Issue an error if pme_order < fourier_nx/y/z
I found by a random test that if pme_order is less than the number of cells in a
dimension of the Fourier grid Gromacs crashes. This is probably an invalid
configuration (I know almost nothing about PME), but it is much better to issue
an error in grompp than crashing.
Here is the code
in pme.c function make_bspline_moduli
when order < nmax, the loop below access the array bsp_data beyond its bounds.