DiscreteVariable: Raise an exception if values are not unique

[janezd]

Issue

Closes #6877.

Description of changes

Check that all values are unique; raise an exception if not.

This does not solve any bugs, but reports them earlier.

Includes

• Code changes
• Tests

[markotoplak]

This bug catcher is so good that it successfully reported 2 bugs on the first attempt.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.41%. Comparing base (c6a79f6) to head (dc7b9b2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6878   +/-   ##
=======================================
  Coverage   88.40%   88.41%           
=======================================
  Files         326      326           
  Lines       71946    71979   +33     
=======================================
+ Hits        63605    63641   +36     
+ Misses       8341     8338    -3     

[janezd]

I'm so proud of my dilligent work. :)

However, as I unintentionally wrote "This does not solve any bugs, but reports them earlier.". :)

• Now I reimplemented the code that creates labels for discretized variables so that it increases the number of digits when necessary. When the number of digits is given as an argument (as opposed to using ContinuousVariable.str_val), it also rounds the interval threshold so that they match the printed labels.
• The other bug was due to an invalid test.