NestedSeries Implementation

[dougbrn]

Resolves #304. This PR has gotten large enough that it might have been better to split it up into a few smaller steps, sorry about that. I've yet to write documentation, but I think it makes sense to write it as a follow up PR while we iron out any implementation/api behaviors here.

Some design choices I made here, which we can definitely do differently:

• NestedSeries takes on a set of the nest accessor functions for direct use
• NestedSeries works with non-nested dtypes just as a normal pandas series, but nested specific properties/methods are tagged with a decorator which will throw an exception for attempted use with a non-nested dtype
• When returning a non-nested series, still try to return a native pandas series. Don't use NestedSeries as an everywhere replacement for pandas series when it's not needed.
• For masking, return result as a NestedSeries always, instead of sometimes as a NestedFrame

[github-actions]

Before [919fe82]	After [8b9c68f]	Ratio	Benchmark (Parameter)
1.21±0.02ms	1.34±0ms	1.1	benchmarks.NestedFrameReduce.time_run
705±10ms	745±100ms	1.06	benchmarks.ReadFewColumnsS3.time_run
10.9±0.03ms	11.4±0.2ms	1.05	benchmarks.NestedFrameAddNested.time_run
10.5±0.1ms	10.8±0.1ms	1.02	benchmarks.NestedFrameQuery.time_run
64.8±0.3ms	65.5±0.9ms	1.01	benchmarks.CountNestedBy.time_run
177M	179M	1.01	benchmarks.ReadFewColumnsHTTPS.peakmem_run
250M	250M	1	benchmarks.AssignSingleDfToNestedSeries.peakmem_run
135M	135M	1	benchmarks.CountNestedBy.peakmem_run
102M	102M	1	benchmarks.NestedFrameAddNested.peakmem_run
107M	106M	1	benchmarks.NestedFrameQuery.peakmem_run

Click here to view all benchmarks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.19%. Comparing base (46acb8e) to head (adc5874).
⚠️ Report is 145 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #331      +/-   ##
==========================================
+ Coverage   98.11%   98.19%   +0.08%     
==========================================
  Files          18       19       +1     
  Lines        1748     1830      +82     
==========================================
+ Hits         1715     1797      +82     
  Misses         33       33              

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[hombit]

Looks great, thank you!

[gitosaurus]

LGTM!