Added unit tests for reaction isomorphism.
Added unit tests for reaction isomorphism.

Currently all but the 2-to-3 ("AB=CDE") tests pass. 
faribas/RMG-Py,comocheng/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,faribas/RMG-Py,enochd/RMG-Py,pierrelb/RMG-Py,pierrelb/RMG-Py,nickvandewiele/RMG-Py,comocheng/RMG-Py,enochd/RMG-Py,KEHANG/RMG-Py,KEHANG/RMG-Py,nyee/RMG-Py