Support for other packages for VDW

[chrisiacovella]

In addition to TAD-DFTD3, support the nvalchemi-toolkit-ops version.

• Small refactoring in the code to accept different "schemes" (or library) in the .toml file and make this toggle happen in the code.
• TAD-DFTD3 provided some utilities to batch the inputs (to be in the form they expect). NVALCHEMI should be able to handle batched data in the same way we are exporting from our neighbor list, but this needs to be investigated more thoroughly to ensure calculations are correct and implemented in an efficient way.