added a method for labeledAtoms info retrieval
added a method for labeledAtoms info retrieval

given reactants and products, this method could return
reactants and products with labeledAtoms.

pierrelb/RMG-Py,nickvandewiele/RMG-Py,nickvandewiele/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py