Fix handling of duplicate variableids

[mikapfl]

Closes: #28

Minimal changes to fix the handling of duplicate variable IDs. This entails changes to the public
API! In particular, UNFCCCSingleCategoryApiReader.variables now has a generic index
instead of using the variableId as index. Also, the query function now
correctly restricts queries if category_ids are provided and correctly fills
all categories with data for a multi-category variable.

This should lead to more correct and complete data returned in some situations.

NB: I also checked again if the same data is returned when using different category_ids for the same category, and no. The two "other" categories (with or without lulucf) are very different. I hope it is correct to return both as the same category when querying? I don't know, though.

[codecov]

Codecov Report

Merging #31 (65c87e6) into main (8348b0a) will increase coverage by 0.45%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   92.82%   93.27%   +0.45%     
==========================================
  Files           3        3              
  Lines         209      223      +14     
==========================================
+ Hits          194      208      +14     
  Misses         15       15              

Impacted Files	Coverage Δ
unfccc_di_api/unfccc_di_api.py	91.80% <93.33%> (+0.47%)	⬆️
unfccc_di_api/tests/test_unfccc_di_api.py	100.00% <100.00%> (ø)

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