Sort _hipscat_index column

[delucchi-cmu]

Change Description

Closes #179, closes #180

Solution Description

• Explicitly either creates a new hipscat index, uses an existing hipscat index, or doesn't do anything with hipscat_index.
• Clumps the functionality under fewer conditionals so the flow of the method is cleaner.
• Sets pandas index to hipscat_index when using existing values and sorts.

[codecov[bot]]

Codecov Report

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

Project coverage is 99.75%. Comparing base (b571ef8) to head (4f1b3ef). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #243 +/- ## ======================================= Coverage 99.75% 99.75% ======================================= Files 24 24 Lines 1203 1204 +1 ======================================= + Hits 1200 1201 +1 Misses 3 3 ```

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