fix indexing and update pandas code per v3.0 changes

[nkasica]

This PR fixes breaking changes from Pandas 3.0.0:

• Default of df.groupby(..., observed=False) changed to df.groupby(..., observed=True)
• Copy-on-Write is now default
  • This deprecates the need for defensive copies
  • Also disallows chained assignment
• Changes to indexing assignment or apply() seemed to break lines 284 and 285

Note: Pandas 3.0.0 now has Python minimum version >= 3.11

Resolves: Issue #153

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.20%. Comparing base (afb4b69) to head (374d979).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #154      +/-   ##
===========================================
+ Coverage    69.15%   69.20%   +0.05%     
===========================================
  Files           14       14              
  Lines         1744     1747       +3     
===========================================
+ Hits          1206     1209       +3     
  Misses         538      538              

Flag	Coverage Δ
unittests	69.20% <100.00%> (+0.05%)	⬆️

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