Add Operator versions of DiagonalStack and VerticalStack

[bwohlberg]

Add Operator versions of DiagonalStack and VerticalStack. Resolves #326.

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (e378d7a) 94.82% compared to head (c78c86b) 94.83%.

Files	Patch %	Lines
scico/operator/_stack.py	89.77%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #477      +/-   ##
==========================================
+ Coverage   94.82%   94.83%   +0.01%     
==========================================
  Files          90       91       +1     
  Lines        5618     5651      +33     
==========================================
+ Hits         5327     5359      +32     
- Misses        291      292       +1     

Flag	Coverage Δ
unittests	94.83% <91.26%> (+0.01%)	⬆️

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