Tally trigger

This is the OpenMC code with tally precision triggers and corresponding documentation. There are some changes in the tally_header file. This Pull request fulfills issue #264.
So you just added the word ``trigger`` to the names for these new subroutines? How about adding it to the beginning of each subroutine name, such as ``trigger_tally_statistics`` and ``trigger_statistics_result`` to keep a little more continuity?