MED3 support, final bits: field write. This brings MED3 support up to par with MED2.

Caveats:

- We don't support new MED3 features (evolving meshes, multi-profile per field step (?))
- I still get crashes on files that used to work with MED2. However the mdump code
  shipped with MED3 also crashes on these files, so the bug might not be with Gmsh...