Make tune_pme understand the "-nsteps" option. Fixes #1650

All command line arguments that tune_pme does not understand are passed over
to mdrun. For a quick sanity check the mdrun options are used together with the
additional option "-nsteps 1", which then appears twice on the command line.

The solution is to let tune_pme understand the "-nsteps" argument, so that it
only occurs on the mdrun command line at launch time. The option for that was
already there but before called "-simsteps".

Change-Id: I8362ced75e67b0e28b4a5e1bd427ae6ae4e0e758