Modularize code structure

[gallego-posada]

Closes #48

Changes

⚠️ This is a backwards-compatibility breaking change!

ConstrainedMinimizationProblem (CMP)

• Remove is_constrained argument/attribute from CMP.
• All logic branching that depended on is_constrained has been refactored inside each CooperOptimizer subclass (see below).

Constrained Optimizer

• Create CooperOptimizer class.
• Create separate classes for different kinds of optimizers. For example SimultaneousConstrainedOptimizer, AlternatingConstrainedOptimizer, ExtrapolationConstrainedOptimizer.
• Create explicit UnconstrainedOptimizer for wrapping (primal) optimizers when dealing with unconstrained problems.
• The user has to select explicitly which kind of update to perform and choose the corresponding ConstrainedOptimizer subclass.
• This change removes much "if/else" branching that existed in the previous ConstrainedOptimizer class, and makes maintaining the code for the individual optimizers simpler.
• Add utility function cooper.optim.constrained_optimizers.utils.create_optimizer_from_kwargs so users can conveniently create the "right" optimizer class based on their choice of alternating/extrapolation/etc.
• Add cooper.optim.constrained_optimizers.cooper_optimizer.CooperOptimizerState dataclass for storing the state of primal and dual optimizers, and other configurations of a CooperOptimizer object.

Formulation

• Create separate module formulation

Multipliers

• Create separate module multipliers