Check MPI calls within OpenMP parallelized regions
Currently we require the MPI library to provide the threading levels "MPI_THREAD_SERIALIZED" or "MPI_THREAD_MULTIPLE". This is done for safety and might not be necessary for all cases of calling ELPA.
- make a list of all MPI calls (also from subroutines) which are called from within OpenMP parallel regions
- check for all calls, whether it can be guaranteed which thread (master or any) will initiate the communication and which thread (master, or the same who initiated the call, or any) can end the communication
- adapt the required threading level accordingly