Implement NDCG@K measure (K=5,10). Fixes #72

[osma]

No description provided.

[coveralls]

Coverage increased (+0.02%) to 96.548% when pulling 4dcf48c on ndcg-at-k into f35b645 on master.

[codecov]

Codecov Report

Merging #73 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   96.52%   96.54%   +0.01%     
==========================================
  Files          27       27              
  Lines        1037     1043       +6     
==========================================
+ Hits         1001     1007       +6     
  Misses         36       36

Impacted Files	Coverage Δ
annif/eval.py	98.11% <100%> (ø)	⬆️
tests/test_eval.py	100% <100%> (ø)	⬆️

---

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 f35b645...4dcf48c. Read the comment docs.