Various improvements

[bwohlberg]

Split admm and pgm modules into multiple source files for easier editing, and various other minor improvements.

[codecov]

Codecov Report

Merging #290 (2b35bde) into main (e2a9a4e) will increase coverage by 0.06%.
The diff coverage is 96.11%.

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
+ Coverage   94.15%   94.21%   +0.06%     
==========================================
  Files          49       53       +4     
  Lines        3269     3303      +34     
==========================================
+ Hits         3078     3112      +34     
  Misses        191      191              

Flag	Coverage Δ
unittests	94.21% <96.11%> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
scico/linop/_circconv.py	83.83% <0.00%> (ø)
scico/linop/_convolve.py	86.31% <ø> (ø)
scico/linop/_diff.py	92.68% <ø> (ø)
scico/linop/_matrix.py	95.86% <ø> (ø)
scico/optimize/_ladmm.py	98.86% <ø> (ø)
scico/optimize/_primaldual.py	100.00% <ø> (ø)
scico/optimize/_pgm.py	93.54% <93.54%> (ø)
scico/optimize/_admmaux.py	95.00% <95.00%> (ø)
scico/optimize/_admm.py	97.41% <97.41%> (ø)
scico/optimize/_pgmaux.py	99.12% <99.12%> (ø)
... and 2 more

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[Michael-T-McCann]

This itstat initialization is always a stumbling block for me when reading the class. Could it be easily refactored into its own function/class?

[bwohlberg]

Perhaps. It would probably have to be some sort of private auxiliary class rather than a general utility, but that's worth considering. I'll make a separate issue as a reminder.