Tests for charge neutrality when atomtyping

[mattwthompson]

Atomtyping a (neutral) molecule to have non-zero total charge should fail atomtyping tests.

[summeraz]

Charge neutrality is already being checked if saving from an mBuild Compound. However, since Foyer supports atomtyping of more than just mBuild Compounds, perhaps removing these lines of code from mBuild and sticking them into Forcefield.apply() before the Structure is returned would be a better solution.

[ctk3b]

sticking them into Forcefield.apply() before the Structure is returned would be a better solution

That sounds great 👍

[mattwthompson]

This is an ancient idea but I think still worth implementing. I can't think of any changes in the past 2 years that have made this obsolete or conflict with it.