Add optional relaxation to ADMM solver

[bwohlberg]

• Add optional relaxation to ADMM solver.
• Simplify ADMM step implementation.

[codecov]

Codecov Report

Merging #118 (e12109c) into main (76ad408) will increase coverage by 0.35%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   90.49%   90.84%   +0.35%     
==========================================
  Files          42       42              
  Lines        2987     2983       -4     
==========================================
+ Hits         2703     2710       +7     
+ Misses        284      273      -11     

Flag	Coverage Δ
unittests	90.84% <100.00%> (+0.35%)	⬆️

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Impacted Files	Coverage Δ
scico/admm.py	95.23% <100.00%> (+5.60%)	⬆️

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[Michael-T-McCann]

I suspect the code was written this way so that the z_and_u_step could be jitted (jitted functions should be pure, no side effects). However, z_and_u_step was not actually jitted as far as I can tell. So I don't mind the refactor.

[bwohlberg]

That was my suspicion too. If we decide there is some benefit to jitting, we can refactor again, but for now this simpler structure makes more sense.