Update atomtype function in utils.py

[rmatsum836]

Describe the behavior you would like added to Foyer
In #296 we ran into odd behavior running test_trappe.py with the atomtype function. (These systems are a hybrid of UA and atomistic) If non_atomistic is set to True, then the element attribute of the atoms is changed and causes a atomic number / symbol confusion. The current fix for these hybrid atomistic/ UA systems is to set non_atomistic=False

Describe the solution you'd like
Update the atomtype function so that the non_atomistic argument is accurate and works for these hybrid UA/atomistic systems.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.