new GridTools::flatten_triangulation

New GridTools::flatten_triangulation. Gives the ability to copy a flattened version of a Triangulation, with possibly different spacedims.

> I think it's conceptually similar to extrude_triangulation, merge_triangulations. Of course, extract_boundary_mesh is in GridTools... :-(

I don't know. If you think it belongs to GridGenerator, I can move it there...