Added merge() and split() methods to Molecule class.
Added merge() and split() methods to Molecule class.

Along with copy(), these are useful when manipulating the graphs in memory.

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