Update num_classes in jaccard score to be a required argument

[SkafteNicki]

What does this PR do?

Fixes #829

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[codecov]

Codecov Report

Merging #853 (1cd919c) into master (b0bcccb) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #853   +/-   ##
=====================================
  Coverage      95%    95%           
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  Files         167    167           
  Lines        6904   6904           
=====================================
  Hits         6562   6562           
  Misses        342    342