Issue188 lazy result conversion

[osma]

Fixes #188

[codecov]

Codecov Report

Merging #198 into master will decrease coverage by 0.17%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   98.76%   98.58%   -0.18%     
==========================================
  Files          44       44              
  Lines        1862     1911      +49     
==========================================
+ Hits         1839     1884      +45     
- Misses         23       27       +4

Impacted Files	Coverage Δ
tests/test_backend_fasttext.py	100% <ø> (ø)	⬆️
annif/backend/tfidf.py	100% <100%> (ø)	⬆️
tests/test_backend.py	100% <100%> (ø)	⬆️
annif/backend/dummy.py	100% <100%> (ø)	⬆️
annif/backend/http.py	100% <100%> (ø)	⬆️
annif/backend/fasttext.py	98.8% <100%> (ø)	⬆️
annif/util.py	97.36% <100%> (ø)	⬆️
tests/conftest.py	100% <100%> (ø)	⬆️
tests/test_backend_http.py	100% <100%> (ø)	⬆️
tests/test_eval.py	100% <100%> (ø)	⬆️
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2ab107f...265248b. Read the comment docs.