Cleanup documentation and fix a few minor things

[bhazelton]

Description

This fixes a number of minor issues:

• Add information about citing pyuvsim via the JOSS paper to the README
• Decrease the tolerance for analytic diffuse tests and add comments explaining the choices
• Clarify the deprecation process in the README
• Replace the utils.strip_extension function with a call to os.path.splitext

I also updated the CI yamls to use pytest-xdist where possible to try to speed it up.

Motivation and Context

fixes #361
fixes #325
fixes #268
fixes #381

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Reference simulation update or replacement
• Documentation change (documentation changes only)
• Version change
• Build or continuous integration change
• Other: Refactoring and test improvements.

Checklist:

For all pull requests:

• I have read the contribution guide.
• My code follows the code style of this project.

Documentation change checklist:

• Any updated docstrings use the numpy docstring format.
• If this is a significant change to the readme or other docs, I have checked that they are rendered properly on ReadTheDocs. (you may need help to get this branch to build on RTD, just ask!)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (abb8a87) 99.55% compared to head (7afb6ac) 99.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #448      +/-   ##
==========================================
- Coverage   99.55%   99.55%   -0.01%     
==========================================
  Files          12       12              
  Lines        2239     2229      -10     
==========================================
- Hits         2229     2219      -10     
  Misses         10       10              

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[jpober]

Looks good, thanks!