[FIX] OWPythagorasTree: Enable node selection from forests with categorical variables.

[thocevar]

Issue

Fixes #3414

Description of changes

Transform data to the discretized domain when computing the samples in leaves (SklTreeAdapter).

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3775 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3775      +/-   ##
==========================================
+ Coverage   84.98%   85.04%   +0.05%     
==========================================
  Files         374      374              
  Lines       69114    69094      -20     
==========================================
+ Hits        58738    58762      +24     
+ Misses      10376    10332      -44

[pavlin-policar]

This fixes the crash. You also figured out why the tooltips are wrong for the random forest trees, but I suppose that belongs in a separate PR.

[pavlin-policar]

I hate to be annoying, but this test really belongs to the tree widget tests, not the forest widget tests. If the behaviour is in the tree widget, the tests should be added to the tree widget tests. Otherwise, the tests for the forest widget are also testing the tree widget, which would quickly become messy.