[FIX] OWDistributions: Fix binning of meta attributes

[janezd]

Issue

File -> Select columns -> Distributions

Load Iris, move one of the (continuous) attributes to meta attributes and select it in the Scatter plot. Instead of that attribute, the scatter plot shows the class.

The first problem is that the attribute was selected incorrectly - as the first attribute in domain + metas. When the grouping attribute was an ordinary attribute and the shown attribute a meta, this resulted in showing the grouping attributes.

The second problem is that discretization classes do not discretize meta attributes.

Description of changes

Construct the table for discretization so that the attribute that has to be discretized always appears as a normal attribute.

Includes

• Code changes
• Tests

[codecov-io]

Codecov Report

Merging #2068 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2068      +/-   ##
=========================================
+ Coverage   69.75%   69.8%   +0.04%     
=========================================
  Files         315     315              
  Lines       53925   53936      +11     
=========================================
+ Hits        37615   37648      +33     
+ Misses      16310   16288      -22

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