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. 
Do you mean that I should use a dictionary or set for N_SCORE_MAX or find_score? When reading input file, the code must first know the filter and scores then it can define where the score for trigger is ? And this part of source code is for reading filters. At this time, the code has no idea how many triggers will be set for scores