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.
Can you leave out ``METHOD`` in ``VARIANCE_METHOD``, ``RELATIVE_ERROR_METHOD``, etc.? This is not a very informative name. If the names are available, change these to ``VARIANCE``, ``RELATIVE_ERROR`` and ``STANDARD_DEVIATION``. Otherwise, I think ``VARIANCE_TRIGGER``, ``RELATIVE_ERROR_TRIGGER`` and ``STANDARD_DEVIATION_TRIGGER`` are more clear and informative names.