feat: add ZenodoReader which doesn't rely on the UNFCCC Api

[mikapfl]

The ZenodoReader fetches data from zenodo instead of the UNFCCC API and presents a similar interface.

[review-notebook-app]

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[codecov]

Codecov Report

Merging #79 (529ec4b) into main (48650f5) will increase coverage by 0.15%.
The diff coverage is 95.91%.

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   93.83%   93.98%   +0.15%     
==========================================
  Files           3        3              
  Lines         227      266      +39     
==========================================
+ Hits          213      250      +37     
- Misses         14       16       +2     

Impacted Files	Coverage Δ
unfccc_di_api/unfccc_di_api.py	92.41% <92.00%> (-0.10%)	⬇️
unfccc_di_api/__init__.py	100.00% <100.00%> (ø)
unfccc_di_api/tests/test_unfccc_di_api.py	100.00% <100.00%> (ø)

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[JGuetschow]

Looks good to me.
I would add comments about the partial implementation.
And I don't understand how we will deal with data updates on zenodo