Support for online learning

[osma]

Part of #225 and #230. WIP. Needs CLI and REST API functionality too.

[codecov]

Codecov Report

Merging #257 into master will decrease coverage by 0.03%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage   99.28%   99.24%   -0.04%     
==========================================
  Files          51       51              
  Lines        2503     2518      +15     
==========================================
+ Hits         2485     2499      +14     
- Misses         18       19       +1

Impacted Files	Coverage Δ
tests/test_backend_vw_multi.py	100% <100%> (ø)	⬆️
annif/backend/vw_multi.py	100% <100%> (ø)	⬆️
annif/backend/backend.py	94.11% <50%> (-2.76%)	⬇️

---

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[codecov-io]

Codecov Report

Merging #257 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
+ Coverage   99.27%   99.31%   +0.03%     
==========================================
  Files          52       52              
  Lines        2499     2629     +130     
==========================================
+ Hits         2481     2611     +130     
  Misses         18       18

Impacted Files	Coverage Δ
annif/project.py	100% <100%> (ø)	⬆️
annif/cli.py	99.43% <100%> (+0.02%)	⬆️
annif/backend/vw_multi.py	100% <100%> (ø)	⬆️
annif/backend/dummy.py	100% <100%> (ø)	⬆️
tests/test_cli.py	100% <100%> (ø)	⬆️
annif/corpus/document.py	100% <100%> (ø)	⬆️
tests/test_backend_vw_multi.py	100% <100%> (ø)	⬆️
annif/exception.py	87.5% <100%> (+1.13%)	⬆️
tests/test_corpus.py	100% <100%> (ø)	⬆️
tests/test_rest.py	100% <100%> (ø)	⬆️
... and 7 more

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[osma]

Note to self: the CLI and REST tests now try to perform online learning with the tfidf backend, but it doesn't (yet - see #226) really support it. There should be a mechanism for a backend to state whether it supports online learning or not, and the CLI and REST operations should report whether the operation succeeded or not depending on what the backend supports.

[osma]

This pull request introduces 1 alert when merging 5b9d7df into 131b993 - view on LGTM.com

new alerts:

• 1 for Unused local variable

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Comment posted by LGTM.com

[osma]

This pull request introduces 1 alert when merging bdef18f into 68e0f43 - view on LGTM.com

new alerts:

• 1 for Unused local variable

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Comment posted by LGTM.com

[osma]

REST side now done. CLI still needs to indicate whether learning succeeded or not.