Refactor SubjectSet and Document to store subject IDs instead of URIs and labels

[osma]

This PR refactors the SubjectSet class, which are used to represent gold standard / manually indexed subjects for training and evaluation documents, so that the SubjectSet class only stores numeric subject IDs instead of subject URIs and labels. It also changes the Document class to contain a SubjectSet instead of separate fields for subject URIs and labels. The end result is that internally, numeric IDs are used much more than before and the conversion from concept URIs and/or labels to subject IDs is performed earlier than before. The changes should simplify things overall and also likely improve efficiency (both RAM and CPU), although I haven't measured the difference.

This PR is very similar to PR #604 which did the same kind of overhaul for the SubjectSuggestion class.

[codecov]

Codecov Report

Merging #606 (9f152c7) into master (391ed7c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #606      +/-   ##
==========================================
- Coverage   99.55%   99.55%   -0.01%     
==========================================
  Files          86       86              
  Lines        5673     5663      -10     
==========================================
- Hits         5648     5638      -10     
  Misses         25       25              

Impacted Files	Coverage Δ
annif/backend/pav.py	98.88% <ø> (-0.02%)	⬇️
annif/corpus/combine.py	100.00% <ø> (ø)
annif/project.py	99.38% <ø> (-0.01%)	⬇️
tests/test_eval.py	100.00% <ø> (ø)
annif/backend/dummy.py	100.00% <100.00%> (ø)
annif/backend/ensemble.py	100.00% <100.00%> (ø)
annif/backend/fasttext.py	100.00% <100.00%> (ø)
annif/backend/mllm.py	100.00% <100.00%> (ø)
annif/backend/nn_ensemble.py	99.29% <100.00%> (-0.01%)	⬇️
annif/backend/omikuji.py	97.46% <100.00%> (-0.10%)	⬇️
... and 20 more

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
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No Coverage information
0.0% Duplication