Tools for dataframes-to-series convertion

[hombit]

This PR fixes #45 introducing few ways to convert a sequence of "light curves" to a nested series:

• packer.pack_dfs() is renamed to pack_seq. It is reimplemented with NestedExtensionArray.from_sequence() which expends it from dataframe-items to things like None, pd.NA, {"time": [...], "flux": [...]}, pa.scalar(...), etc. Now it also supports index and dtype arguments
• packer.pack() which is a wrapper around both pack_flat() for pd.DataFrame input and pack_seq() for everything else
• NestedFrame.add_nested() now calls pack() instead of pack_flat() to support sequence inputs.

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[github-actions]

Before [109c16c]	After [65e0429]	Ratio	Benchmark (Parameter)
4.26±0.3s	3.36±0.7s	~0.79	benchmarks.time_computation
2.29k	3.54k	1.55	benchmarks.mem_list

Click here to view all benchmarks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.50%. Comparing base (109c16c) to head (442321b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
+ Coverage   97.35%   97.50%   +0.14%     
==========================================
  Files          14       14              
  Lines         794      801       +7     
==========================================
+ Hits          773      781       +8     
+ Misses         21       20       -1     

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[dougbrn]

Looks good!