Fix atomtypeTest for inert gases (Ar, He, Ne)
Fix atomtypeTest for inert gases (Ar, He, Ne)

We had previously wanted these to return None as an atom type.
Now apparently we have atom types for them. (I hope we weren't
relying on a None value anywhere). At least these now pass.

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