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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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