Temporarily store labeled atoms when generating reactions.
Temporarily store labeled atoms when generating reactions.

This is done instead of copying the reactants in order to minimize the
number of deep copies of molecules, which are expensive.

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