[hxt] verify that all 2D elements are triangles

See merge request !479

HXT only supports triangles, and we want the user to know it (cfr. #1462)

corresponding MVertex* maps are invalidated by renumbering if sorted by tag; just sort using the pointers (fix for #2153)

- only pad physical name to multiples of MED_TAILLE_LNOM lengths if there is more than 1 physical group per entity

fix issue #2132

See merge request !478

When the starting tet for the *walk* is a ghost tetrahedron (a tetrahedron with the ghost vertex), we go inside the neighbor tet that is not a ghost tet instead.
I forgot to check if that tet was inside our partition.