refactor reducer

[hillalex]

• Ahead of adding configurable data scale, refactors root state to include a dictionary of dataset settings
• Pulls out reducers into their own files
• Also factors out ChooseDataset as a common component, to be used in the sidebar as well as the index page.

[codecov]

Codecov Report

Attention: Patch coverage is 95.04132% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.93%. Comparing base (20f23e8) to head (813a34d).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/App.tsx	0.00%	3 Missing ⚠️
src/reducers/datasetReducer.ts	93.54%	2 Missing ⚠️
src/hooks/usePersistedState.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
+ Coverage   86.74%   86.93%   +0.19%     
==========================================
  Files          15       18       +3     
  Lines         362      398      +36     
  Branches       73       74       +1     
==========================================
+ Hits          314      346      +32     
- Misses         48       52       +4     

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