[FIX] Variable: Prevent hashing of Values of DiscreteVariable.

[thocevar]

Issue

Values of StringVariable are hashed incorrectly (fixes #2912).

Values of DiscreteVariable are impossible to hash as dictated by the __eq__ function. For example:

from Orange.data.variable import *
val = Value(DiscreteVariable("var", ["red","green","blue"]), 1)
print(val == 1, hash(val) == hash(1))  # True True
print(val == "green", hash(val) == hash("green"))  # True False

Description of changes

As we can not make hash(1) == hash("green"), we prevent hashing of Values of DiscreteVariables with an exception. Users should hash directly ids or domain values instead.

This might break things that use dictionaries or sets of Values. However, the tests didn't discover any such cases.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #3217 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #3217      +/-   ##
=========================================
+ Coverage    82.8%   82.8%   +<.01%     
=========================================
  Files         346     346              
  Lines       59601   59603       +2     
=========================================
+ Hits        49350   49355       +5     
+ Misses      10251   10248       -3

[brunap]

Lan, this change disables plotting in Biopharm add-on: ProcessPlot widget.