Tally trigger

This is the OpenMC code with tally precision triggers and corresponding documentation. The trigger can be set for combined k_effective and tallies. The types of trigger can be set to relative error, standard deviation and variance. In some situation, it can predict the number of batches to satisfy the trigger. As far as  I know, there's no problem. Hope everyone can give advice and guidance. This Pull request fulfills issue #264. 
I have change the line 511  --> "OpenMC includes tally precision triggers which allow the user to define  uncertainty thresholds on :math:`k_{eff}` in :ref:`eigenvalue` and/or other 
tallies in :ref:`tally`."