Add ADMM subproblem solver for matrix operators

[bwohlberg]

Add ADMM subproblem solver for matrix operators, addressing one of the remaining components of #232.

Also:

• Clean up implementations of block convolution subproblem solvers.
• Improvements to Diagonal linop.
• Minor fixes to docs and exception types.
• Add two utility functions.

[codecov]

Codecov Report

Merging #426 (a7843e8) into main (894f8b3) will increase coverage by 0.08%.
The diff coverage is 94.86%.

@@            Coverage Diff             @@
##             main     #426      +/-   ##
==========================================
+ Coverage   94.78%   94.86%   +0.08%     
==========================================
  Files          86       86              
  Lines        5309     5426     +117     
==========================================
+ Hits         5032     5147     +115     
- Misses        277      279       +2     

Flag	Coverage Δ
unittests	94.86% <94.86%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
scico/linop/_matrix.py	96.00% <ø> (ø)
scico/optimize/_admm.py	96.00% <ø> (ø)
scico/optimize/admm.py	100.00% <ø> (ø)
scico/optimize/_admmaux.py	96.81% <89.36%> (+1.50%)	⬆️
scico/solver.py	97.40% <95.40%> (-1.26%)	⬇️
scico/linop/_diag.py	97.62% <100.00%> (+1.26%)	⬆️
scico/util.py	92.95% <100.00%> (+0.48%)	⬆️

... and 1 file with indirect coverage changes

[Michael-T-McCann]

Couple of suggestions.