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 mean you should make find_score a dictionary/set local to the subroutine in input_xml where it is used.