Fix PME order and grid size checks

Fix PME order and grid size checks

The PME grid size restrictions were not checked in grompp.
Now both grompp and mdrun check the grid size. Both checks are done
with the same minimum grid size independent of DD and OpenMP settings.
Renamed calc_grid to calcFftGrid and let it take the minimum grid size
into account.
Also corrected an assert that checked for pme_order>=4 iso 3.

Change-Id: I57b300f4a413ea2942fa671be67839be7ae16c39