PME load balancing now checks for PME grid restrictions

To enable this, the PME restrictions checks have been moved
to a separate function called gmx_pme_check_restrictions.
Also allowed nkx==nnodes_major*(order-1) again with threading.

Change-Id: I50b6f8cfaeba5e360c534424c80c320515812e43
Merge "PME load balancing now checks for PME grid restrictions" into release-4-6