Add "Column as Model"

[janezd]

Issue

Closes #6813.

Description of changes

A widget that "predicts" classes from a single column. I have several questions.

• The widget offers
  • discrete columns whose values are the same, or a subset of, class values,
  • and numeric columns if the class is binary.
• When using numeric columns, its values are used as probabilities of class with index 1 (in which case they must be between 0 and 1) or mapped through logistic function with user-specified offset and coefficient.
• Widget also outputs a model, so it can be fed into Test Learner and compared with other models.

I've put the widget into category Evaluate. This is not a model but rather a trick to turn a Table into Evaluation Results, hence it belongs there because any user interested in this transformation would look for a widget in this category.

Thoughts?

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Attention: Patch coverage is 98.90110% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.29%. Comparing base (70ebf27) to head (62d47f2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6852      +/-   ##
==========================================
+ Coverage   88.27%   88.29%   +0.02%     
==========================================
  Files         326      328       +2     
  Lines       71137    71319     +182     
==========================================
+ Hits        62793    62972     +179     
- Misses       8344     8347       +3