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.
is there a reason you use ``t1``, ``t2`` and ``t3`` instead of ``variance``, ``rel_err`` and ``std_dev``? It seems to me that these names would be much easier for others to understand and follow in the code.