Relationship of IfcGradientCurve.EndPoint, IfcSegmentedReferenceCurve.EndPoint with zero-length segment

As noted by @RickBrice https://github.com/IfcOpenShell/IfcOpenShell/pull/3982/files#diff-be0a3e3e00f8d8a24a29c355bd022eb663c4310e27251191374b9aa6d05ab4b4R100

This seems redundant.

> If the geometry definition is also present, then each of the zero-length segments shall have a [IfcCurveSegment](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcCurveSegment.htm) counterpart - of length zero.

https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/concepts/Product_Shape/Product_Geometric_Representation/Alignment_Geometry/Alignment_Geometry_-_Segments/content.html

Should we deprecate these EndPoint attributes, as:

- not consistent with business logic layer, where we can only have zero-length segments
- not consistent with compcurve for horizontal, which doesn't have this attribute

CC @evandroAlfieri @peterrdf