Generalize atoms clamping

[CedricTravelletti]

As described in #1 (comment) , one should be able to restrict atoms movement along a submanifold, e.g. certain atoms are allowed to slide on a surface.
Currently atoms are either completely clamped or not.

Possible implementation directions include:

• Using constraints in OptimizationFunction (see docs).
• Using optimization on manifolds: Manopt.jl.

[cortner]

there is a question whether this should be part of Geometry optimization or part of AtomsBase. Is the constraint part of the system or part of the simulation? E.g. I can see that one might want to do other kinds of simulation tasks with the same constraints....

[rkurchin]

I vaguely recall a conversation to this effect on one of the Zoom calls (or maybe it was at JuliaCon hackathon last summer?). If I had to pick one, I think I'd say it's part of the simulation, but it's probably really a separate thing and if we could make it "portable" in the sense you describe, that would be ideal.

[cortner]

Maybe the AtomsBase interface should be extended for managing constraints but not necessarily provide implementations of constraints or just very simple examples.

I am also very unsure about this tbh.