[FIX] Predictions widget: handle similar but different domains

[jerneju]

Issue

Widget throws an error similar to "index 1 is out of bounds for axis 0 with size 1". That occurs when predictions which are sent to Predictions widget are made from a data with different domain than a domain which is also sent to that widget. For instance, predictions predict only two possible target values but the data has also a third value.
https://sentry.io/biolab/orange3/issues/223474227/
https://sentry.io/biolab/orange3/issues/216151441/

Description of changes

Predictions table is extended to match the number of the data target's values.

Includes

• Code changes
• Tests
• Documentation

[jerneju]

https://sentry.io/biolab/orange3/issues/223474227/

[jerneju]

https://sentry.io/biolab/orange3/issues/216151441/

[markotoplak]

I think it is misleading to show probabilities of zero for a class that did not exist in the train data. Only the probabilities for classes present in the train data should be shown.

Furthermore, even if the class of the test data is completely different than the class of the train data (all the values are different) the widget should still work. Perhaps a warning should be shown in this case.

[janezd]

Could you consider this PR in the light of #2183, which slightly changes the meaning of the widget's attribute class_var to represent the (common!) class of predictors? Are the two PRs in any conflict (not in terms of code, but does merging of one break the other?).

If it does, I'd prefer merging #2183 first, since it fixes a more critical bug.

[markotoplak]

It seems that with changes from #2183 the problem does not appear anymore.

[codecov-io]

Codecov Report

Merging #2129 into master will decrease coverage by 0.85%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2129      +/-   ##
==========================================
- Coverage   72.02%   71.17%   -0.86%     
==========================================
  Files         318      318              
  Lines       54580    54691     +111     
==========================================
- Hits        39309    38924     -385     
- Misses      15271    15767     +496

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Too complicated for now.