TODO

General:

1. xicsrt.objects._XicsrtOpticMesh:
    Mesh refinement and pre-selection currently uses two mesh intersect
    methods that accept different array dimensions. Right now mesh_intersect_1
    uses an optimized algorithm, but mesh_intersect_2 uses an older non-optimized
    version.  At least mesh_intersect_2 needs to be updated to the better
    algorithm; it would be even better if the two could be combined somehow.

2. xicsrt.objects._XicsrtOpticMesh:
    The docstring is out of date, and needs to be rewritten.

3. xicsrt.objects._GeometryObject:
    The vectors zaxis and xaxis should be checked for orthogonality. If not
    orthogonal an Exception should be raised. Behavior for non-orthogonal
    coordinate systems is not defined.