Restore missing BlockArray docs

[bwohlberg]

A side of effect of #259 was removal of the detailed BlockArray docs (see readthedocs). Fixing this required

1. Moving scico.numpy.blockarray to scico.numpy._blockarray since the blockarray = BlockArray.blockarray line in scico/numpy/__init__.py overwrote the use of blockarray as a submodule name.
2. Moving the detailed docs into a Sphinx .rst file since there were additional difficulties in including the docs from scico.numpy._blockarray.

Some attention is still required, as indicated in comments to be added after creation of the PR.

[codecov]

Codecov Report

Merging #285 (eef8cea) into main (85b9942) will increase coverage by 0.00%.
The diff coverage is 97.56%.

@@           Coverage Diff           @@
##             main     #285   +/-   ##
=======================================
  Coverage   94.15%   94.15%           
=======================================
  Files          49       49           
  Lines        3267     3269    +2     
=======================================
+ Hits         3076     3078    +2     
  Misses        191      191           

Flag	Coverage Δ
unittests	94.15% <97.56%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
scico/scipy/__init__.py	100.00% <ø> (ø)
scico/numpy/_blockarray.py	97.33% <97.33%> (ø)
scico/numpy/__init__.py	100.00% <100.00%> (ø)
scico/numpy/_wrappers.py	97.10% <100.00%> (ø)
scico/numpy/util.py	97.46% <100.00%> (ø)
scico/scipy/special.py	100.00% <100.00%> (ø)

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[Michael-T-McCann]

Thanks for catching this and fixing it!