Speed up optimize command and avoid high memory usage

[osma]

Fixes #236

[codecov]

Codecov Report

Merging #238 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   98.58%   98.62%   +0.03%     
==========================================
  Files          48       48              
  Lines        2121     2178      +57     
==========================================
+ Hits         2091     2148      +57     
  Misses         30       30

Impacted Files	Coverage Δ
annif/hit.py	95.32% <100%> (+1.2%)	⬆️
annif/cli.py	96.98% <100%> (ø)	⬆️
annif/eval.py	100% <100%> (ø)	⬆️
tests/test_hit.py	100% <100%> (ø)	⬆️

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