Move optimizers into optimize package

[bwohlberg]

Move optimizers into optimize package. Resolves #122.

Notes:

• solvers not moved into optimize to avoid import loops
• admm and pgm submodules are treated differently from _ladmm and _primaldual because they include auxiliary classes for which the retention of the submodule prefix is desirable, e.g. admm.LinearSubproblemSolver
• class ADMM can be imported from either scico.optimize or scico.optimize.admm, and similarly for the PGM solver classes

[codecov]

Codecov Report

Merging #129 (f341f08) into main (4e88014) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
+ Coverage   91.31%   91.34%   +0.02%     
==========================================
  Files          46       47       +1     
  Lines        3214     3223       +9     
==========================================
+ Hits         2935     2944       +9     
  Misses        279      279              

Flag	Coverage Δ
unittests	91.34% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
scico/optimize/_ladmm.py	97.64% <ø> (ø)
scico/optimize/_primaldual.py	98.68% <ø> (ø)
scico/optimize/__init__.py	100.00% <100.00%> (ø)
scico/optimize/admm.py	95.21% <100.00%> (ø)
scico/optimize/pgm.py	94.81% <100.00%> (ø)

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[Michael-T-McCann]

Despite the slight awkwardness you mentioned around LinearSubproblemSolver, this looks like a significant improvement in clarify.