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.
I do think that you should print more information here, such as the tally ID / score / nuclide. The reason is that a user may want to change (reduce) the trigger threshold for whichever tally has the largest number of predicted batches. 
Without this detailed information, the user won't know which tally trigger to adjust.