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.
My guess is @paulromano may want this to be some sort of a warning message rather than an error message, but I may be wrong. It seems to me that the simulation should run till it reaches ``max_batches`` without an error message (but instead a warning message to report the predicted number of batches), even if the predicted number of batches is far greater than ``max_batches``.