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.
Ooops, I meant to say ``satisfy_triggers = .false.``. The ``amount`` variable is just the number of tallies/scores/nuclides where the uncertainty > threshold, correct? So if you ever increment ``amount > 1`` then you automatically know that the triggers are not yet satisfied?