Allow MPI datatype to be reused in remote neighbor update when transferring all cells at once

[iljah]

When transferring all cells' data in the same MPI call dccrg has to create a new MPI datatype representing all cells. Currently this is done every time a remote neighbor update is requested but if none of the cells' datatypes change the superdatatype could be reused. The API for this would probably be to check at compile time whether the cell data class has a static member function such as bool reuse_all_mpi_datatypes(). If one exists query it whether to reuse the superdatatype or not and if it doesn't exists always create a new superdatatype.