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  The reason why I write this line is that we want the n_batches in the last statepoint file produced is the number of batch where the calculation is terminated. Will said he needed to use n_batch to do something, but I forgot what his purpose. @wbinventor  Your idea?