An abstract base class for creating a curve object that contains methods for interpolation.
		For an array of Curves see [page:CurvePath].
		
		[example:webgl_geometry_extrude_splines geometry / extrude / splines ]
		Constructor
		[name]()
		
		This constructor creates a new [name].
		
		Methods
		[method:Vector getPoint]( t )
		Returns a vector for point t of the curve where t is between 0 and 1. Must be implemented in the extending class.
		[method:Vector getPointAt]( u )
		Returns a vector for point at relative position in curve according to arc length
		[method:Array getPoints]( divisions )
		Get sequence of points using getPoint( t ) 
		[method:Array getSpacedPoints]( divisions )
		Get sequence of equi-spaced points using getPointAt( u )
		[method:Float getLength]()
		Get total curve arc length
		[method:Array getLengths]( divisions )
		Get list of cumulative segment lengths
		[method:null updateArcLengths]()
		Update the cumlative segment distance cache
		[method:Float getUtoTmapping]( u, distance )
		Given u ( 0 .. 1 ), get a t to find p. This gives you points which are equidistant
		[method:Vector getTangent]( t )
		Returns a unit vector tangent at t. If the subclassed curve do not implement its tangent derivation, 2 points a small delta apart will be used to find its gradient which seems to give a reasonable approximation
		[method:Vector getTangentAt]( u )
		Returns tangent at equidistant point u on the curve
		Source
		[link:https://github.com/mrdoob/three.js/blob/master/src/[path].js src/[path].js]