Restructure package

[bhazelton]

Description

Restructure the package using the src layout, as in pyradiosky.

I also found some code in the tests handling versions of pyuvdata that we no longer support, so I did some cleanup, including removing one old test data file that is no longer used.

Motivation and Context

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
• [x] Build or continuous integration change

Checklist:

For all pull requests:

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

Build or continuous integration change checklist:

• [ ] If required or optional dependencies have changed (including version numbers), I have updated the readme to reflect this.
• [ ] If this is a new CI setup, I have added the associated badge to the readme and to references/make_index.py (if appropriate).

[codecov[bot]]

Codecov Report

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

Project coverage is 99.95%. Comparing base (bd5c789) to head (7564503).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #455 +/- ## ========================================== + Coverage 99.86% 99.95% +0.09% ========================================== Files 12 12 Lines 2161 2161 ========================================== + Hits 2158 2160 +2 + Misses 3 1 -2 ```

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