Move properties from ProximalOperators to ProximalCore

[hakkelt]

This PR moves property functions (is_smooth, is_convex, is_generalized_quadratic, etc.) from ProximalOperators to this package. Some of the functions are also renamed for clarity:

• is_singleton -> is_singleton_indicator
• is_cone -> is_cone_indicator
• is_affine -> is_affine_indicator
• is_set -> is_set_indicator
• is_prox_accurate -> is_proximable

My goal with that change is that I can reference properties in ProximalAlgorithms such that it can be queried from StructuredOptimizations.jl, what properties are required for the given inputs. E.g.:

import ProximalAlgorithms: get_assumption, FastForwardBackwardIteration, AssumptionGroup, SimpleTerm

get_assumptions(::Type{<:FastForwardBackwardIteration}) = AssumptionGroup(
    SimpleTerm(:f => (is_smooth, is_convex)),
    SimpleTerm(:g => (is_proximable, is_convex,))
)

It allows StructuredOptimization to automatically parse problems for a given algorithm as get_assumptions tells the name of the input (f and g in this case), and exposes enough information for decisions during parsing.

[hakkelt]

Can this PR be merged, or is there anything I can do?

[lostella]

@hakkelt so the overall plan is:

• have the default definitions here
• implement them for specific functions in ProximalOperators
• use them in the solver decision logic in StructuredOptimization

Is this accurate?

[hakkelt]

Exactly

[lostella]

Thanks @hakkelt !

[hakkelt]

Thank you for the review! :)