Simplify .nest["field"] = value

[hombit]

Previously, series.nest["field"] = value would assign both "flat" value and "list" value. This PR drops it to simplify logic and interface of the accessor. List-arrays still may be assigned using series.nest.set_list_field("field_name", list_arrays).

It also makes series.nest["field"] (__getitem__) to use seiries.nest.get_flat_series

Fixes #16

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[github-actions]

Before [136f2c5]	After [3b875c7]	Ratio	Benchmark (Parameter)
1.63k	1.9k	1.16	benchmarks.mem_list
3.83±1s	3.51±1s	0.91	benchmarks.time_computation

Click here to view all benchmarks.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 93.27%. Comparing base (136f2c5) to head (5cf131d).

Files	Patch %	Lines
src/nested_pandas/series/accessor.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   92.93%   93.27%   +0.34%     
==========================================
  Files          13       13              
  Lines         566      565       -1     
==========================================
+ Hits          526      527       +1     
+ Misses         40       38       -2     

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[dougbrn]

Looks good!