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
• [x] Documentation change (documentation changes only)
• [ ] Version change
• [ ] Build or continuous integration change
• [x] Other: Refactoring and test improvements.

Checklist:

For all pull requests:

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

Documentation change checklist:

• [x] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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