[FIX] Gradient Descent: work on sparse

[jerneju]

Issue

Does not work on sparse.

Description of changes

Convert those two or three columns which are needed to dense.

Includes

• [X] Code changes
• [x] Tests
• [ ] Documentation

[jerneju]

https://sentry.io/biolab/orange3-educational/issues/417311830/

[codecov-io]

Codecov Report

Merging #45 into master will increase coverage by 0.18%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   91.24%   91.42%   +0.18%     
==========================================
  Files          14       14              
  Lines        1782     1785       +3     
  Branches      214      215       +1     
==========================================
+ Hits         1626     1632       +6     
+ Misses        120      118       -2     
+ Partials       36       35       -1

Impacted Files	Coverage Δ
...gecontrib/educational/widgets/owgradientdescent.py	81.79% <100%> (+0.89%)	:arrow_up:

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[PrimozGodec]

Can you just resolve conflicts? @jerneju

[PrimozGodec]

Thank you @jerneju