[ENH] k-Means: Output centroid labels

[janezd]

Issue

Fixes #3687.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3695 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3695      +/-   ##
==========================================
+ Coverage   84.49%   84.51%   +0.01%     
==========================================
  Files         373      373              
  Lines       68268    68317      +49     
==========================================
+ Hits        57683    57736      +53     
+ Misses      10585    10581       -4

[lanzagar]

The output table with centroids has no name (was like that before this PR already). Can we think of something (anything) and set that attribute (so that e.g. Source from the Concatenate widget output makes more sense)?
I am fine with any naming scheme, maybe something like:
name (centroids), centroids - name, ...
where name = self.data.name

[janezd]

I went for "name centroids", for instance "iris centroids".

I didn't write a separate test but just added assertions to existing tests. Test do not mock clustering (I won't add mocking within this PR - I tried but it's not trivial), so separate tests would slow down the testing suite.