Refactoring of `mpipack_pmmg.h` and `mpiunpack_pmmg.h` to use `MPI_Struct` instead of char array.

[Algiane]

The mesh packing inside a char array implies that we need large intergers to store the message size.

An alternative that was implemented in the past (and was working I think) used hierarchical MPI_Struct to define the C structures used in the mesh (points, xpoints, tetra...) as well as the mesh itself. It should allow to easier the mesh communication and to reduce the size of the integer used to provide the message size (because the MPI structures will be larger than chars which is the minimal unit for the type of variables).

See issue #113 and PR#117