[ENH] Outliers: Save model into compute_value

[VesnaT]

Issue

Outlier widget could be used for novelty detection using the Apply Domain widget.

Description of changes

• invoking model's call operator now returns original dataset, appended an Outlier meta variable
• save fitted model into the Outlier variable compute_value
• the data output of Outliers widget can be used as a template data for Apply Domain widget
• fix EllipticEnvelopeClassifier for dataset with missing values

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4372 into master will increase coverage by <.01%.
The diff coverage is 95.15%.

@@            Coverage Diff             @@
##           master    #4372      +/-   ##
==========================================
+ Coverage   87.13%   87.13%   +<.01%     
==========================================
  Files         399      399              
  Lines       72901    72960      +59     
==========================================
+ Hits        63521    63577      +56     
- Misses       9380     9383       +3