Clarification of the ELPA usage: It has always been intended with the ELPA API that one should only set the MPI communicators ("mpi_comm_parent", "mpi_comm_rows", and "mpi_comm_cols") once per ELPA object. Technically, it has been possible to change these communicators, for an existing ELPA object, which leads -- dependent on the exact configuration of the ELPA object -- to correct or erroneous behaviour.
With this commit, it is technically impossible to set the communicators more than once (per ELPA object)