Add support for approximate isotropic TV norm

[bwohlberg]

Add support for approximate isotropic TV norm.

[codecov]

Codecov Report

Attention: Patch coverage is 96.61017% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 94.65%. Comparing base (d9026d3) to head (05e7da1).
Report is 1 commits behind head on main.

❗ Current head 05e7da1 differs from pull request most recent head d428572. Consider uploading reports for the commit d428572 to get more accurate results

Files	Patch %	Lines
scico/functional/_tvnorm.py	96.36%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #507      +/-   ##
==========================================
- Coverage   94.67%   94.65%   -0.02%     
==========================================
  Files          91       91              
  Lines        5707     5741      +34     
==========================================
+ Hits         5403     5434      +31     
- Misses        304      307       +3     

Flag	Coverage Δ
unittests	94.65% <96.61%> (-0.02%)	⬇️

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