add fix for from_flat

[dougbrn]

Resolves #138 .

[github-actions]

Before [6e2f433] <v0.2.0>	After [a035380]	Ratio	Benchmark (Parameter)
1.11±0.02ms	1.13±0.01ms	1.02	benchmarks.NestedFrameReduce.time_run
277M	282M	1.02	benchmarks.ReassignHalfOfNestedSeries.peakmem_run
86.9M	87.1M	1	benchmarks.NestedFrameAddNested.peakmem_run
10.0±0.2ms	10.0±0.1ms	1	benchmarks.NestedFrameAddNested.time_run
92M	92.1M	1	benchmarks.NestedFrameQuery.peakmem_run
6.72±0.09ms	6.71±0.1ms	1	benchmarks.NestedFrameQuery.time_run
90.5M	90.6M	1	benchmarks.NestedFrameReduce.peakmem_run
263M	261M	0.99	benchmarks.AssignSingleDfToNestedSeries.peakmem_run
32.4±1ms	31.5±0.9ms	0.97	benchmarks.AssignSingleDfToNestedSeries.time_run
63.8±3ms	60.9±3ms	0.95	benchmarks.ReassignHalfOfNestedSeries.time_run

Click here to view all benchmarks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.46%. Comparing base (6e2f433) to head (0556592).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #139   +/-   ##
=======================================
  Coverage   99.46%   99.46%           
=======================================
  Files          14       14           
  Lines         932      933    +1     
=======================================
+ Hits          927      928    +1     
  Misses          5        5           

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