Fix training SVC on fulltext corpus

[juhoinkinen]

Claudia reported in annif-users email list that they encountered problems when trying to train the SVC backend on full-text corpus. Their initial problem was not the one that this PR fixes, but their report brought this up.

Training SVC backend on fulltext corpus does not work but fails with TypeError: 'set' object is not subscriptable. This is because DocumentDirectory defines uris for documents as a set, while in SVC it was assumed that uris are list or other subscriptable (which for DocumentFile is true).

This PR simply makes sure the uris are a list. If a document has multiple uris a NotSupportedException is raised, because a set is not ordered so a random uri from the ones defined for the training document would be taken (if there are many).

To make SVC training unit tests work I made a clumsy fixture document_corpus_single_subject that uses the regular document_corpus fixture but includes only one subject for each document. This could surely be improved in some way.

I will also make an issue for the discrepancy of uri container types (set from from DocumentDirectory vs list from DocumentFile).

[codecov]

Codecov Report

Merging #501 (efced0a) into master (2b8e3c9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #501   +/-   ##
=======================================
  Coverage   99.48%   99.49%           
=======================================
  Files          78       78           
  Lines        5672     5687   +15     
=======================================
+ Hits         5643     5658   +15     
  Misses         29       29           

Impacted Files	Coverage Δ
annif/backend/svc.py	100.00% <100.00%> (ø)
tests/conftest.py	100.00% <100.00%> (ø)
tests/test_backend_svc.py	100.00% <100.00%> (ø)

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[sonarcloud]

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