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. 
I added PR #319 that removes `n_batches` from the statepoint files.  If that PR is accepted, then we can just remove this line.  Otherwise, I think the easiest solution is to make `max_batches` a global variable and then use the number of digits from that variable in the zero padding.