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.
Perhaps I am wrong, but it appears that none of these defined types have underscores ``_`` in their names except for yours - ``Temp_trig`` and ``K_trigger``. Perhaps the style guide calls for purely camel case names for all defined types, in which case ``TempTrig`` and ``KTrig`` are better names? @paulromano can you comment on this?