Feature/devel newmesh

This branch introduces the distributed mesh file pmesh.nc in replacement of mesh_information.nc. Designed for and tested on a single MPI process as a firsts step. More work is needed to make XIOS I/O and halo exchanges work with >1 MPI process. Along the way some changes have been made to type t_geometry, see remarks in geometry.f90. A separate module has been introduced for type t_cellset.