using Mesh_triangulation_3 (which is an enriched Regular_triangulation_3)
compiles, but the calls of tr.is_valid() fail since the triangulation
is not regular anymore after remeshing
we reconvert it back to the demo c3t3 triangulation type anyhow, to be able
to display it, hoping that is_valid() will not be called later on
- make it work with Regular_triangulation and its Weighted_points
- remove everything which was not needed about Surface_patch_index
(1 is in the complex, 0 is not, and is_in_complex(facet) tests
incident subdomains)