Add block diagonal linear operator

[Michael-T-McCann]

Looking for feedback: is the name appropriate? We haveBlockDiagonalLinearOperator and LinearOperatorStack, which don't seem parallel and have the awkward "LinearOperator" part.

closes #311

[codecov]

Codecov Report

Merging #315 (4644b38) into main (7562063) will decrease coverage by 0.09%.
The diff coverage is 83.72%.

@@            Coverage Diff             @@
##             main     #315      +/-   ##
==========================================
- Coverage   94.03%   93.93%   -0.10%     
==========================================
  Files          54       54              
  Lines        3384     3411      +27     
==========================================
+ Hits         3182     3204      +22     
- Misses        202      207       +5     

Flag	Coverage Δ
unittests	93.93% <83.72%> (-0.10%)	⬇️

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

Impacted Files	Coverage Δ
scico/linop/_stack.py	87.64% <82.50%> (-2.69%)	⬇️
scico/linop/__init__.py	100.00% <100.00%> (ø)
scico/linop/_diff.py	98.30% <100.00%> (ø)

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[bwohlberg]

BlockDiagonalLinearOperator is consistent with other names in use, but it's quite the mouthful. Perhaps replace LinearOperator with LinOp in long names, or something like renaming LinearOperatorStack to LinearOperatorVStack and BlockDiagonalLinearOperator to LinearOperatorDStack?