Discrete variable: remove ordered attribute

[PrimozGodec]

Issue

As we already discussed in #4793 ordered attribute from the Discrete variable is not respected in most of the widgets anymore (except Feature Statistics), so it can be removed. Order defined in values tuple should be respected in any widget which needs order.

Description of changes

• The ordered attribute is deprecated and will stay so until 3.28.0 then it can be removed.
• Checkbox which sets ordered attribute in the edit domain is removed
• Feature statistic will never show min and max value for the discrete variable (before it happened in a case when ordered attribute was True)
• When saving data always write the order of values for discrete in the header (except in the case with only one value)

Includes

• Code changes
• Tests

[codecov]

Codecov Report

Merging #4818 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4818      +/-   ##
==========================================
- Coverage   84.19%   84.18%   -0.02%     
==========================================
  Files         282      277       -5     
  Lines       57355    56472     -883     
==========================================
- Hits        48292    47541     -751     
+ Misses       9063     8931     -132     

[janezd]

This is a bit dangerous: any call that passes compute_value as non-keyword argument will (possibly silently) fail. What about keeping ordered as an argument, but changing its default to None and the test if "ordered" in kwargs: to if ordered is not None?

Though: this is unlikely and, besides, such calls won't be fixed until the argument is actually removed and the calls will actually fail. We ignore warnings, including those about deprecations.

[PrimozGodec]

You are right. Didn't think about this possibility. This solution was to avoid problems with __repr__ but now I fixed it differently.