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. 
The number of batches already simulated is definitely needed to compute statistics. However, the number of batches for the full run, `n_batches`, doesn't strictly need to be there. It's just one piece of data that's always been included.