Interface seems to be missing documentation

[cortner]

Similarly as with AtomsBase I find the specification of what exactly is the interface could be made clearer. In my case I missed the promote_force_type discussion. It's kind of there but very hidden and it really isn't clear that this is something that a new calculator should implement. Just an example.

No rush to address this until the interface has settled a bit but eventually I would appreciate a discussion how to make this as simple and as clear as possible.