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. 
@paulromano @wbinventor I have solved it. BTW there's something wrong with the tests of openmc from mit-crpg/openmc. I run the tests and few failed. I run the very new openmc from mit-crpg, and the result doesn't agree with the .dat file. And I think I can't pay much time on github for the internet is unstable. Hope you can merge is as soon as possible