Add a Portable Model Driver (PMD) interface

[ellio167]

Add a Portable Model Driver (PMD) interface that would allow simulators to directly use a MD and send it (in a programatic way) a directory with any necessary parameter files. This would allow MDs to be used without an explicit (compiled) PM.

For example,LAMMPS could (in principle) replace its pair lj/cut implementation with a use of a KIM PMD.

• Will need to define a component of the PMD-API that allows a PMD to specify the list of parameter files needed and for a simulator to "register" those files to be used with a PMD object/interface.

• If we can also create a "standard" extension for LAMMPS. Then it might be feasible to have all new LAMMPS "pair styles" be created and managed as KIM PMD's. The extension would allow access to LAMMPS specific features/data that is not currently supported by the kim-api. The idea would be to create new PMDs to be as conformant as possible to the kim-api, but still be able to use non-kim-api features in LAMMPS. This would allow these items to be part of the KIM ecosystem and would help identify those features that would be most impactful for inclusion in future versions of the kim-api. Similar "standard" extensions could potentially be created for other simulators (DL_POLY, GAP, ASE, etc.).