Determine --procsPerNode automatically

[vasdommes]

Currently --procsPerNode is passed by user.

However, in MPI-3 we can determine it using shared memory communicator:

MPI_Comm shared_memory_comm;
MPI_Comm_split_type(MPI_COMM_WORLD, MPI_COMM_TYPE_SHARED, 0, MPI_INFO_NULL, &shared_memory_comm);
int procsPerNode = El::mpi::Rank(shared_memory_comm);

NB: we should also assert that each node has the same number of processes, otherwise the block distribution algorithm will fail.