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 have the same general comment as before: We shouldn't be allocating anything to the same size as the results array. That array might be huge for some simulations, so this could potentially blow out memory. Why can't you check triggers one by one without allocating space for all?