Create and use MPI wrappers to encapsulate most of the MPI functionality

Description

Now the wrappers from mpi_debug_oct_m have been moved to mpi_oct_m and are type-bound procedures of the mpi_grp_t type. They can be simply used as mpi_grp%gatherv(...) without the need for passing the communicator and the error code. The wrappers include calls to the debugging facility in mpi_debug_oct_m which has been restructured to avoid circular dependencies.

News snippet

Create and use MPI wrappers to encapsulate most of the MPI functionality

Checklist

• I have checked that my code follows the Octopus coding standards