Move FairChem to external model posture

[CompRhys]

Following #120 we want to move more models to external posture to gradually reduce maintenance burden. For the time being we will till wrap the packages here even with external implementations.

[CompRhys]

@zulissimeta is this something that fairchem would be happy to upstream? We will likely keep the legacy interface around for a bit longer. We would also likely wrap the interface you implement here to reduce any breaking changes (but we're not that opposed to breaking changes to improve functionality whilst this is still a developing code).