Add optimize package

[Michael-T-McCann]

With the addition of new optimizers, I find having each optimizer as a top-level module odd. I.e., looking at the table of contents for the API reference (https://scico--117.org.readthedocs.build/en/117/_autosummary/scico.html), we have scico.admm and scico.ladmm at the same level as scico.linop. I propose that it's time for a scico.optimizer package.

Originally posted by @Michael-T-McCann in #117 (comment)

From in person discussion: Names should be like scico.optimize.ADMM. scico.solver functions should move over to scico.optimize.

[bwohlberg]

Copied from discussion in #117:

That's a good suggestion, but there are a few questions to resolve regarding the implementation:

• Naming:
  • Instead of optimizer, I would go with optimize, for consistency with scipy.optimize.
  • Are we going to also move scico.solver into this subpackage? If not, the name should perhaps reflect specific properties that differentiate it from the functions in scico.solver, e.g. scico.proxopt, since all the algorithms that would be within the subpackage would be proximal algorithms.
• Structure. Do we
  • retain the existing modules, so that e.g. scico.admm.ADMM becomes scico.optimize.admm.ADMM, or
  • follow the same approach as in e.g.scico.linop, so that scico.admm.ADMM becomes scico.optimize.ADMM.

My leaning is towards naming it scico.optimize, which would include scico.solver, and making the classes direct members of this module, e.g. scico.optimize.ADMM.