Non-orthorhombic simulation boxes with LAMMPS

[wutobias]

Description
Please describe the behavior you would like added to Interchange.

When building LAMMPS input using to_lammps with non-orthrombic simualation boxes, an error is raised NotImplementedError: Interchange does not yet support exporting non-rectangular boxes to LAMMPS. I'd like to use LAMMPS for simulations of non-orthorombic crystal lattices, hence the possibility to export these kind of simulation boxes would be nice. I am attaching an OFF topology as an example (.json serialized) off-topology.json.zip.

Below is code I'd like to use

forcefield = ForceField("openff-2.0.0.offxml")
interchange = forcefield.create_interchange(
        topology, charge_from_molecules=topology.molecules)
interchange.to_lammps("input.inp")

[mattwthompson]

This only isn't supported because I didn't think people would need it, nice to see that's not the case

I might not get to this for a few weeks - would be happy to take a PR if you're up for it. The change would need to go here, just converting the (OpenMM-style) box vectors into whichever format LAMMPS needs.

[mattwthompson]

cc @timbernat

[wutobias]

Thanks. I'll try to get a PR together (unless @timbernat wants to go ahead). From the documentation, it seems that LAMMPS's restricted triclinic box form is similar to the reduced form that OpenMM uses.

[timbernat]

@wutobias I'd love to take a crack at this but won't have the bandwidth to do so until the week of 11/18, so I'm happy for you to take the reins on this