Issue with C3t3 IO

[janetournois]

We have another issue to fix. When I uncomment the calls to writing the mesh in the example mesh_polyhedral_complex_sm.cpp, and feed out.mesh into the example c3t3_example.cpp, and then load after_remeshing.mesh into CGALlab.exe many border facets are missing:

Originally posted by @afabri in #8793 (comment)