grompp does not generate PME grid size 84
If the fourier-spacing chosen in the mdp file is too fine for a grid of size 80, grompp will skip to 96, ignoring the perfectly fine value of 84. This results in high FFT and FFT communication costs.
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.