MPI communicators are not freed when reading or writing restarts

[atrayano]

In MAPL2, we use a derived type "ArrDescr" to hold additional information needed for reading or writing restarts. This type contains 6 MPI communicators, most of which are set by calling MPI_Comm_Split. These communicators are not freed at the end of the IO operation, and as a consequence, a very long run with frequent writing checkpoints would fail (most likely due to exhausting some internal MPI limit)