Fits str columns to utf8

[Schwarzam]

This PR closes #281

• Resolves string columns remaining with b'*' (bytes sign) when converting fits to pandas.

I added a string column "test_id" to tests/hipscat_import/data/test_formats/small_sky.fits

Code Quality

• [x] I have read the Contribution Guide
• [x] My code follows the code style of this project
• [x] My code builds (or compiles) cleanly without any errors or warnings

[codecov[bot]]

Codecov Report

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

Project coverage is 99.35%. Comparing base (53e9d24) to head (0c4d591).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #285 +/- ## ======================================= Coverage 99.35% 99.35% ======================================= Files 24 24 Lines 1234 1238 +4 ======================================= + Hits 1226 1230 +4 Misses 8 8 ```

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