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.
change message to something like:

``message =  "The estimated number of batches is "//trim(to_str(n1)) for "&
                   // trim(trig_dis % temp_name) //" in tally "// trim(to_str(trig_dis % id))``