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.
Based on what I've seen in the code, ``TempTrig`` might be better named ``TriggerDistance``. What do you think?

Also, you have a lot of variables with ``temp`` in the name - this isn't a big deal, but I think it is more distracting than informative in the name. If possible, either remove it or come up with something more accurate.