CDT_3 and Tetrahedral remeshing - missing API glue

[janetournois]

Issue Details

In the output of CDT and get_remeshing_triangulation(), each facet has its own Surface_patch_index.
When plc_face_id is given, some coplanar facets may be merged into one surface patch, but the other facets have their own patch ids.

Then, Tetrahedral_remeshing will treat all the patch (or facets) boundary edges as patch borders, and keep them (they can be resampled, but endpoints and geometry are kept).

If the user wants to keep the input edges as CDT does, then it's good. But if the user is fine with remeshing and modifying his surface mesh, we should add an option let tetrahedral remeshing flip them, for example.

In CGAL Lab, I added "reset surface patches" on a CDT, to have all surface facets with the same surface ID.
That's nice for smooth objects, but looses the patch, and hence feature edges, information, that we may want to keep.

We need an extra layer of API to merge surface patches into larger patches at the end of CDT.