Move feature extraction from TF-IDF backend to project. Fixes #80

[osma]

No description provided.

[codecov]

Codecov Report

Merging #83 into master will decrease coverage by 1.51%.
The diff coverage is 85.6%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   96.69%   95.17%   -1.52%     
==========================================
  Files          28       29       +1     
  Lines        1149     1161      +12     
==========================================
- Hits         1111     1105       -6     
- Misses         38       56      +18

Impacted Files	Coverage Δ
annif/backend/tfidf.py	100% <100%> (ø)	⬆️
tests/test_backend_tfidf.py	100% <100%> (ø)	⬆️
annif/util.py	100% <100%> (ø)
annif/corpus/__init__.py	100% <100%> (ø)	⬆️
tests/test_cli.py	100% <100%> (ø)	⬆️
annif/corpus/subject.py	86% <78.12%> (-14%)	⬇️
annif/project.py	91.72% <79.24%> (-8.28%)	⬇️

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[coveralls]

Coverage decreased (-1.5%) to 95.177% when pulling dd2b7e5 on project-feature-extraction into ba04aa4 on master.