Lightweight “base” package with core definitions

[lostella]

Similar to other packages, we could have a very lightweight “base” package containing the core definitions (like prox and prox!, maybe some calculus rules would fit there too), so that other packages like ProximalAlgorithms or SeparableOptimization do not need to take the full dependency on ProximalOperators, but can just depend “on the API”. This would increase decoupling between packages: anyone bringing their own operators could directly use available algorithms.

Some things to figure out:

• What should fit into the package? Definitely prox and prox! but other things like function conjugation would be useful I think
• What would be a good name? ProximalBase is taken. Maybe ProximalCore?