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.
Misspelled "minimum." Change "N_SCORE_MINUM" to "N_SCORE_MINIMUM."

And what is this for? The comment "expand the boundary" isn't clear to me, but perhaps that is just because I am not as familiar with the OpenMC source code as you are.