Add support for OrbMol and move Orb to external model posture

[thomasloux]

#373 Added support for spin and charge for MLIP on molecules for Mace and Fairchem. It needs to be added as well for Orbitals model OrbMol

[CompRhys]

I am increasingly in favor of trying to move to the external model posture (#120) now ts seeing a bit more uptake. I swapped nequip framework to do this recently (#400) and rejigged the tests to use the existing interface check code which was partially duplicated (#401).

Do the orbital team feel like we're mature enough here to take the implementation over there? @timduignan, @vsimkus, @benrhodes26

We probably need to extend/add another interface validation function for spins and charges.

[vsimkus]

@CompRhys Apologies for the late reply. Can confirm that this is now in the works!

[CompRhys]

That's great to hear! I am working on #433 to make it easier to keep the ase comparison tests we have if people are inclined to do so.