Implementation of a [link:http://en.wikipedia.org/wiki/Quaternion quaternion]. This is used for [link:https://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation rotating things] without encountering the dreaded [link:http://en.wikipedia.org/wiki/Gimbal_lock gimbal lock] issue, amongst other advantages.
		var quaternion = new THREE.Quaternion();
		quaternion.setFromAxisAngle( new THREE.Vector3( 0, 1, 0 ), Math.PI / 2 );
		var vector = new THREE.Vector3( 1, 0, 0 );
		vector.applyQuaternion( quaternion );
		
		
		[page:Float x] - x coordinate
		[page:Float y] - y coordinate
		[page:Float z] - z coordinate
		[page:Float w] - w coordinate
		
			Used to check whether this or derived classes are Quaternions. Default is *true*.
			You should not change this, as it is used internally for optimisation.
		
Returns the angle between this quaternion and quaternion [page:Quaternion q] in radians.
Creates a new Quaternion with identical [page:.x x], [page:.y y], [page:.z z] and [page:.w w] properties to this one.
Returns the rotational conjugate of this quaternion. The conjugate of a quaternion represents the same rotation in the opposite direction about the rotational axis.
Copies the [page:.x x], [page:.y y], [page:.z z] and [page:.w w] properties of [page:Quaternion q] into this quaternion.
		[page:Quaternion v] - Quaternion that this quaternion will be compared to.
		Compares the [page:.x x], [page:.y y],	[page:.z z] and [page:.w w] properties of
		[page:Quaternion v] to the equivalent properties of this quaternion to determine if they
		represent the same rotation.
		
Calculates the [link:https://en.wikipedia.org/wiki/Dot_product dot product] of quaternions [page:Quaternion v] and this one.
		[page:Array array] - array of format (x, y, z, w) used to construct the quaternion.
		[page:Integer offset] - (optional) an offset into the array.
		Sets this quaternion's [page:.x x], [page:.y y],	[page:.z z] and [page:.w w] properties
		from an array.
		
Inverts this quaternion - calculates the [page:.conjugate conjugate]. The quaternion is assumed to have unit length.
Computes the [link:https://en.wikipedia.org/wiki/Euclidean_distance Euclidean length] (straight-line length) of this quaternion, considered as a 4 dimensional vector.
Computes the [link:https://en.wikipedia.org/wiki/Euclidean_distance Euclidean length] (straight-line length) of this quaternion, considered as a 4 dimensional vector. This can be useful if you are comparing the lengths of two quaternions, as this is a slightly more efficient calculation than [page:.length length]().
[link:https://en.wikipedia.org/wiki/Normalized_vector Normalizes] this quaternion - that is, calculated the quaternion that performs the same rotation as this one, but has [page:.length length] equal to *1*.
Multiplies this quaternion by [page:Quaternion q].
		Sets this quaternion to [page:Quaternion a] x [page:Quaternion b].
		Adapted from the method outlined [link:http://www.euclideanspace.com/maths/algebra/realNormedAlgebra/quaternions/code/index.htm here].
		
Pre-multiplies this quaternion by [page:Quaternion q].
			[page:Quaternion q] - The target quaternion.
			[page:float step] - The angular step in radians.
			Rotates this quaternion by a given angular step to the defined quaternion *q*.
			The method ensures that the final quaternion will not overshoot *q*.
		
			[page:Quaternion qb] - The other quaternion rotation
			[page:float t] - interpolation factor in the closed interval [0, 1].
			Handles the spherical linear interpolation between quaternions. [page:float t] represents the
			amount of rotation between this quaternion (where [page:float t] is 0) and [page:Quaternion qb] (where
			[page:float t] is 1). This quaternion is set to the result. Also see the static version of the
			*slerp* below.
			
			// rotate a mesh towards a target quaternion
			mesh.quaternion.slerp( endQuaternion, 0.01 );
			
		
Sets [page:.x x], [page:.y y], [page:.z z], [page:.w w] properties of this quaternion.
		Sets this quaternion from rotation specified by [page:Vector3 axis] and [page:Float angle].
		Adapted from the method [link:http://www.euclideanspace.com/maths/geometry/rotations/conversions/angleToQuaternion/index.htm here].
		*Axis* is assumed to be normalized, *angle* is in radians.
		
Sets this quaternion from the rotation specified by [page:Euler] angle.
		Sets this quaternion from rotation component of [page:Matrix4 m].
		Adapted from the method [link:http://www.euclideanspace.com/maths/geometry/rotations/conversions/matrixToQuaternion/index.htm here].
		
		Sets this quaternion to the rotation required to rotate direction vector [page:Vector3 vFrom] to
		direction vector [page:Vector3 vTo].
		Adapted from the method [link:http://lolengine.net/blog/2013/09/18/beautiful-maths-quaternion-from-vectors here].
		[page:Vector3 vFrom] and [page:Vector3 vTo] are assumed to be normalized.
		
		[page:Array array] - An optional array to store the quaternion. If not specified, a new array will be created.
		[page:Integer offset] - (optional) if specified, the result will be copied
		into this [page:Array].
		Returns the numerical elements of this quaternion in an array of format [x, y, z, w].
		
			Static methods (as opposed to instance methods) are designed to be called directly from the class,
			rather than from a specific instance. So to use the static version of, call it like so:
			
THREE.Quaternion.slerp( qStart, qEnd, qTarget, t );
			
			By contrast, to call the 'normal' or instanced slerp method, you would do the following:
			
//instantiate a quaternion with default values
var q = new THREE.Quaternion();
//call the instanced slerp method
q.slerp( qb, t )
			
		
			[page:Quaternion qStart] - The starting quaternion (where [page:Float t] is 0)
			[page:Quaternion qEnd] - The ending quaternion (where [page:Float t] is 1)
			[page:Quaternion qTarget] - The target quaternion that gets set with the result
			[page:float t] - interpolation factor in the closed interval [0, 1].
			Unlike the normal method, the static version of slerp sets a target quaternion to the result of the slerp operation.
			
			// Code setup
			var startQuaternion = new THREE.Quaternion().set( 0, 0, 0, 1 ).normalize();
			var endQuaternion = new THREE.Quaternion().set( 1, 1, 1, 1 ).normalize();
			var t = 0;
			// Update a mesh's rotation in the loop
			t = ( t + 0.01 ) % 1; // constant angular momentum
			THREE.Quaternion.slerp( startQuaternion, endQuaternion, mesh.quaternion, t );
			
		
		[page:Array dst] - The output array.
		[page:Integer dstOffset] - An offset into the output array.
		[page:Array src0] - The source array of the starting quaternion.
		[page:Integer srcOffset0] - An offset into the array *src0*.
		[page:Array src1] - The source array of the target quatnerion.
		[page:Integer srcOffset1] - An offset into the array *src1*.
		[page:float t] - Normalized interpolation factor (between 0 and 1).
		
Like the static *slerp* method above, but operates directly on flat arrays of numbers.
[link:https://github.com/mrdoob/three.js/blob/master/src/[path].js src/[path].js]