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.
Have changed it to  N_SCORE_MAX         =  60    ! This is the index used to find 
                                    ! in which scores trigger is applied,
                                    ! combined with N_SCORE_MINUM 
@wbinventor