IElement required extension methods

1. If the BHoM class implements an IElement interface corresponding with its geometrical representation:

   Interface	Implementing classes
   IElement0D	Classes which can be represented by Point (e.g. nodes)
   IElement1D	Classes which can be represented by ICurve (e.g. bars)
   IElement2D	Classes which can be represented by a planar set of closed ICurves (e.g. planar building panels)

2. It needs to have the following methods implemented in it's oM-specific Engine:

   Interface	Needed methods	Optional methods	When
   IElement0D	Geometry() SetGeometry(Point point)
   IElement1D	Geometry() SetGeometry(ICurve curve)	Elements0D() SetElements0D( List<IElement0D> newElements0D) NewElement0D(Point point)	IElement1D which endpoints are defined by IElement0D
   IElement2D	OutlineElements1D() SetOutlineElements1D( List<IElement1D> outlineElements1D) NewElement1D(ICurve curve) InternalElements2D() SetInternalElements2D( List<IElement2D> internalElements2D) NewInternalElement2D()