Added 'pm7' option to input file parsing for Mopac.
Added 'pm7' option to input file parsing for Mopac.

Also added some more exception handling for undefined methods.
comocheng/RMG-Py,nickvandewiele/RMG-Py,KEHANG/RMG-Py,KEHANG/RMG-Py,chatelak/RMG-Py,enochd/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,enochd/RMG-Py,pierrelb/RMG-Py,faribas/RMG-Py,nickvandewiele/RMG-Py,faribas/RMG-Py,nyee/RMG-Py,pierrelb/RMG-Py,comocheng/RMG-Py