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.
Why do you have the two subroutines ``tally_trigger_statistics`` and ``statistics_trigger_result``? It seems like these should be merged into a single subroutine since ``tally_trigger_statistics`` doesn't really do anything except call ``statistics_trigger_result``.