Refactoring of computational geometry related base classes

[sanbrock]

• Decisions on NXcsg, NXoff_geometry etc. see draft issues here: https://github.com/orgs/nexusformat/projects/4?pane=issue&itemId=62979125
• Bundle cg-related base and contributed in own documentation bucket

This is the list of definitions
NXcg_alpha_complex.nxdl.xml
NXcg_cylinder_set.nxdl.xml
NXcg_ellipsoid_set.nxdl.xml
NXcg_face_list_data_structure.nxdl.xml
NXcg_geodesic_mesh.nxdl.xml
NXcg_grid.nxdl.xml
NXcg_half_edge_data_structure.nxdl.xml
NXcg_hexahedron_set.nxdl.xml
NXcg_marching_cubes.nxdl.xml
NXcg_parallelogram_set.nxdl.xml
NXcg_point_set.nxdl.xml
NXcg_polygon_set.nxdl.xml
NXcg_polyhedron_set.nxdl.xml
NXcg_polyline_set.nxdl.xml
NXcg_primitive_set.nxdl.xml
NXcg_roi_set.nxdl.xml
NXcg_sphere_set.nxdl.xml
NXcg_tetrahedron_set.nxdl.xml
NXcg_triangle_set.nxdl.xml
NXcg_triangulated_surface_mesh.nxdl.xml
NXcg_unit_normal_set.nxdl.xml