Tally trigger

This is the OpenMC code with tally precision triggers and corresponding documentation. The trigger can be set for combined k_effective and tallies. The types of trigger can be set to relative error, standard deviation and variance. In some situation, it can predict the number of batches to satisfy the trigger. As far as  I know, there's no problem. Hope everyone can give advice and guidance. This Pull request fulfills issue #264. 
@paulromano @nhorelik Nick, Paul,
  Today when I run my OpenMC code with trigger function, I find there's bug. If trigger is satisfied, the OpenMC should stopped and output the statepoint file. In fact, if I run it in my computer, without parrellation, it can output the statepoint file, but when I run it on the cluster, it stopped at the "creating statepoint file" that is writing the source point file. I found that the code paused at the write_source_pint() in the engenvalue.F90 (line 290). Could you please help me find what's wrong with the code? Because both I and Will are not familiar with the Fortran Parallel.