Increased the default reset step to 1500 in gmx tune_pme

Increased the default reset step to 1500 in gmx tune_pme

Commit 785aad1a introduced a gmx_fatal() in mdrun for cases where
cycle counters are reset when PME tuning is still active. In
almost all cases, tuning takes longer than 100 steps (which was
the default at which gmx tune_pme would request mdrun to reset its
counters). This leads to gmx tune_pme reporting that all the
runs failed. Note that the small default of 100 steps was from times
where there was only DLB to account for, but not PME tuning.

With the increased default, this should happen only very rarely.

For future versions it would be nicer to implement a "-benchmarksteps"
command line parameter for mdrun which resets counters exactly after
PME tuning is finished and then performs a requested number of
benchmark MD steps. Refs #1781

Change-Id: Icbcce1ecc8a23d35302c04c9a6be13c06b1be8c8