Improve functionality of auto_selfcal pip-installed into monolithic CASA

[psheehan]

This PR is to improve the ability to use auto_selfcal when pip-installed into monolithic CASA. This includes, in particular:

• The ability to install the auto_selfcal command-line tool into the monolithic CASA bin
• The above auto_selfcal command-line tool can be used with mpicasa for parallel processing
• Updated docs to reflect these new/improved capabilities.

@jjtobin, since this includes a documentation update, we should hold off on merging until shortly before our next release so that the docs reflect the capabilities of pip-installed auto_selfcal. Alternatively, if we set readthedocs to make the default 'stable', then I think we could also merge and the updates would only show up on 'latest'. But we would lose the fixes that I made to the docs after the release, so that may not be ideal.

I also want to run the doc changes by you to get your feedback.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 20.00000% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
auto_selfcal/__init__.py	25.00%	12 Missing ⚠️
auto_selfcal/tests/test_auto_selfcal.py	0.00%	4 Missing ⚠️
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Files with missing lines	Coverage Δ
auto_selfcal/tests/test_auto_selfcal.py	62.37% <0.00%> (-1.91%)	⬇️
auto_selfcal/__init__.py	40.00% <25.00%> (-60.00%)	⬇️

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[psheehan]

The full benchmark for this is done running and all tests pass. @jjtobin, I think we can merge this one any time now.