Hierarchical clustering: Allow more than 20 clusters in Top n spin

[janezd]

Issue

Fixes #6883.

Description of changes

The spin box expects some maximum value; Qt's default is 99. I set it to 1000, but the widget will show a warning if the value is above 20.

The warning checks the final number of clusters. It won't be shown if Top N setting is too high, but there are not enough data instances for so many clusters. And the warning is also shown when >20 clusters are obtained in some other way, either manually or via treshold.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.26%. Comparing base (6c742f7) to head (2a6f878).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6887      +/-   ##
==========================================
- Coverage   88.26%   88.26%   -0.01%     
==========================================
  Files         326      326              
  Lines       71184    71188       +4     
==========================================
  Hits        62834    62834              
- Misses       8350     8354       +4