Added autotuning of overlapping communication option

This tends to enable overlapping computation/communication
for long neighbor list build intervals, andt o disable it for short
intervals. The tuning can result in 5-10% speedup
Fix crash in CPU MPI simulations
Remove EOL whitespace

refs #2781
Merge tune_precompute: fixes #2781

# By Jens Glaser (2) and Joshua A. Anderson (1)
* tune_precompute:
  Remove EOL whitespace
  Fix crash in CPU MPI simulations
  Added autotuning of overlapping communication option