tweak GROMACS demo so it doesn't fail when lots of cores are available

[boegel]

Our current GROMACS demo fails when a significant number of cores are available, basically we don't run enough steps:

Fatal error: PME tuning was still active when attempting to reset mdrun counters at step 500.
Try resetting counters later in the run, e.g. with gmx mdrun -resetstep.

We could simply bump the -nsteps 1000, but that's also moving the goal posts, and it will result in runs on a low number of cores to take longer.

Another option could be to try and determine how many cores are available (via the nprocs command?), and scale up the number of step being run based on that (N * 500 steps?).

[boegel]

Any suggestions here @victorusu?

[al42and]

One option is -notunepme to disable PME tuning, since it is unlikely to work well for short runs. Alternatively, one can set -resetstep 100 instead of -resethway, to reset the performance counters earlier, before the PME tuning kicks in.