Add unyt support to custom mixing rules

[rsdefever]

Bug summary
Custom mixing rules do not currently use unyt. We need to reformulate
the custom_mixing_dict to use the unyt package.

[rsdefever]

@rmatsum836 your PR reminded me that custom mixing rules are clunky at the moment. We should see if we can improve them.

[rmatsum836]

Agreed. Currently if custom mixing rules are used, the user must specify all pairwise interactions, even if the majority of them are standard geometric or LB. I think it would be a nice feature to allow the user to only specify the custom interactions, and then infer/write out the other pairwise interactions from the underlying Parmed Structure.