Issue309 key files with labels only

[osma]

Fixes #309

This PR modifies the document corpus processing code so that URIs are looked up based on labels and vice versa (with the help of a subject index). This should make it possible to use simple .key files with only subject labels, as specified in the wiki.

Marking as draft PR since some refactoring may be necessary before merging.

[codecov]

Codecov Report

Merging #313 into master will decrease coverage by 19.38%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master     #313       +/-   ##
===========================================
- Coverage   99.36%   79.97%   -19.39%     
===========================================
  Files          55       55               
  Lines        2842     2867       +25     
===========================================
- Hits         2824     2293      -531     
- Misses         18      574      +556

Impacted Files	Coverage Δ
annif/corpus/convert.py	100% <ø> (ø)	⬆️
annif/corpus/document.py	100% <100%> (ø)	⬆️
tests/test_corpus.py	100% <100%> (ø)	⬆️
annif/corpus/subject.py	100% <100%> (ø)	⬆️
annif/corpus/types.py	100% <100%> (ø)	⬆️
annif/backend/vw_base.py	4.34% <0%> (-95.66%)	⬇️
tests/test_backend_vw_multi.py	4.58% <0%> (-95.42%)	⬇️
annif/backend/mixins.py	0% <0%> (-93.75%)	⬇️
annif/backend/vw_multi.py	6.66% <0%> (-93.34%)	⬇️
annif/backend/fasttext.py	8.21% <0%> (-90.42%)	⬇️
... and 9 more

---

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[osma]

Code Climate keeps complaining but I think the code is now good enough

[codecov]

Codecov Report

Merging #313 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
+ Coverage   99.36%   99.37%   +<.01%     
==========================================
  Files          55       55              
  Lines        2842     2867      +25     
==========================================
+ Hits         2824     2849      +25     
  Misses         18       18

Impacted Files	Coverage Δ
annif/corpus/convert.py	100% <ø> (ø)	⬆️
annif/corpus/document.py	100% <100%> (ø)	⬆️
tests/test_corpus.py	100% <100%> (ø)	⬆️
annif/corpus/subject.py	100% <100%> (ø)	⬆️
annif/corpus/types.py	100% <100%> (ø)	⬆️

---

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[lgtm-com]

This pull request introduces 1 alert when merging 0678998 into 1877f40 - view on LGTM.com

new alerts:

• 1 for Unused import