bugfix.  For +ar and similar pdep reactions, if the pres_mod term is identically zero, then the old system breaks down.  Instead we must actually evaluate k0/kinf
bugfix.  For +ar and similar pdep reactions, if the pres_mod term is identically zero, then the old system breaks down.  Instead we must actually evaluate k0/kinf

SLACKHA/pyJac,kyleniemeyer/pyJac,kyleniemeyer/pyJac,SLACKHA/pyJac