[FIX] Formula: Remove context settings

[ales-erjavec]

Issue

Fixes: gh-5636

Description of changes

Remove context settings from Formula widget.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.24%. Comparing base (8831a57) to head (740b823).
Report is 3 commits behind head on master.

❗ Current head 740b823 differs from pull request most recent head 5c0cf8a

Please upload reports for the commit 5c0cf8a to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6801      +/-   ##
==========================================
- Coverage   88.26%   88.24%   -0.03%     
==========================================
  Files         326      327       +1     
  Lines       71156    71331     +175     
==========================================
+ Hits        62805    62944     +139     
- Misses       8351     8387      +36     

[janezd]

I like the idea, it will solve a lot of problems.

I wrote two formulas, one with petal_length and one with petal_length and petal_width. Then I removed petal_width. All formulas persisted, the widget reported the error (via transform_error, I believe) and did not update the output, so it still output the data from the previous input. I think the widget should do one of the following:

• remove formulae that miss some variables from the model, but keep them in settings as "hints" so they reappear if another input data again contains them,
• show a warning and mark the formulae that cannot be computed (or report them in the warning description) due to missing variables or another problem, but compute the other formulae and update the output,
• report an error (as it does currently, just perhaps without outputting the exception in the console, if possible) and set output to None.