[FIX] Fitter: Properly delegate preprocessors

[pavlin-policar]

Issue

#2091

Includes

• Code changes
• Tests
• Documentation

[pavlin-policar]

This also fixes most of the failing tests in #2092, except the ones that are related to the association rules widget, which isn't a fitter, so it isn't related to this.

[codecov-io]

Codecov Report

Merging #2093 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2093      +/-   ##
==========================================
+ Coverage   69.93%   69.94%   +<.01%     
==========================================
  Files         315      315              
  Lines       53981    53985       +4     
==========================================
+ Hits        37754    37758       +4     
  Misses      16227    16227

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