Clean up straggling instances of hips/hipscat

[delucchi-cmu]

Change Description

• Cleaned up some remaining references to HiPS/HiPSCat.
• Fixed spelling on Hierarchical.
• Removed unit test data READMEs, as their content is now duplicated in the generate_data.ipynb file.

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Codecov Report

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

Project coverage is 92.74%. Comparing base (ab5d272) to head (19ad62f). Report is 2 commits behind head on hats.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## hats #369 +/- ## ======================================= Coverage 92.74% 92.74% ======================================= Files 48 48 Lines 1931 1931 ======================================= Hits 1791 1791 Misses 140 140 ```

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