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.
Add in the simple equation that you use to predict the number of batches. Also, add the following sentence:

"This predictive model assumes no correlation between fission sources distributions from batch-to-batch. This assumption is reasonable for fixed source and small criticality problems, but is very optimistic for highly coupled full-core reactor problems."