First implementation of LSI backend, with tests. Fixes #201

[osma]

Implementation of #201

[codecov]

Codecov Report

Merging #219 into master will decrease coverage by 0.05%.
The diff coverage is 97.5%.

@@            Coverage Diff            @@
##           master    #219      +/-   ##
=========================================
- Coverage   98.65%   98.6%   -0.06%     
=========================================
  Files          46      48       +2     
  Lines        2084    2148      +64     
=========================================
+ Hits         2056    2118      +62     
- Misses         28      30       +2

Impacted Files	Coverage Δ
tests/test_backend_tfidf.py	100% <100%> (ø)	⬆️
annif/backend/__init__.py	100% <100%> (ø)	⬆️
tests/test_backend_lsi.py	100% <100%> (ø)
tests/conftest.py	100% <100%> (ø)	⬆️
annif/backend/lsi.py	95.74% <95.74%> (ø)

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[osma]

Closing this PR for now, see #201 for evaluation results.