Return (bg_size, ranks_per_node) from mpi_procs and omp_threads.

def bgsize_rankspernode(self):
	    """Return (bg_size, ranks_per_node) from mpi_procs and omp_threads."""
	    bg_size = int(math.ceil((self.mpi_procs * self.omp_threads)/ self.hw.cores_per_node))
	    bg_size = max(bg_size, 32) # TODO hardcoded
	    ranks_per_node = int(math.ceil(self.mpi_procs / bg_size))

	    return bg_size, ranks_per_node