Propagate index name in to_flat

[wilsonbb]

Change Description

Currently when we use to_flat on a packed dataframe the name of its original index (which still exists in its pandas series form) is lost. See the following notebook screenshot

This can be fixed by simply having to_flat reference the previous index name when creating its result dataframe.

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[github-actions]

Before [d63cee9]	After [9f36add]	Ratio	Benchmark (Parameter)
3.79±0.5s	2.18±1s	~0.58	benchmarks.time_computation
1.22k	1.94k	1.58	benchmarks.mem_list

Click here to view all benchmarks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.92%. Comparing base (d63cee9) to head (af5951f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #27   +/-   ##
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  Coverage   94.92%   94.92%           
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  Files          13       13           
  Lines         611      611           
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  Hits          580      580           
  Misses         31       31           

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[hombit]

Thank you! TIL about index names