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. 
It looks like you're not catching the default case where the user can specify an MT number for a score. For example, in my tally if I had `<scores>102</scores> ` it would give me the n,gamma reaction rate. A related problem then is that your find_score array is not dimensioned large enough since you have N_SCORE_MAX = 60. Really, you ought to just use a dictionary or a set, which are provided in dict_header and set_header modules.