Added handling of forbidden structures to KineticsGroups class.
Added handling of forbidden structures to KineticsGroups class.

Previously there was a placeholder attribute for this functionality, but
it was ignored. The ForbiddenStructures class makes it straightforward
to implement handling of forbidden structures for reaction families as
well as for the entire database.

nickvandewiele/RMG-Py,enochd/RMG-Py,faribas/RMG-Py,faribas/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py,KEHANG/RMG-Py,comocheng/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,enochd/RMG-Py,KEHANG/RMG-Py,nyee/RMG-Py