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.
Ok, sorry about that. I took a quick look at the original ``input_xml.F90`` last night and thought I saw messages using ``settings.xml`` instead of ``settings XML``, and I wanted your code to be consistent. But if this is how it was written before, then keep it.