Fix calc_grid avoiding PME grid factor 84

Due to an indexing issue, it was unlikely that calc_grid, and thus
grompp, would choose a PME grid of 84*2^n and instead use a factor 96.
For a cubic box, this could increase the FFT cost by a factor 1.5.
Fixes #1605.

Change-Id: I8d6a85a98267af053b66867722fc5626ef907964