correct multiplicity check for molecules without unpaired electrons
correct multiplicity check for molecules without unpaired electrons

Also, made it more clear how we perform the checks

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