[ENH] Heatmap: Restore ability to cluster larger datasets

[ales-erjavec]

Issue

In gh-2000 'Heatmap' widget lost the ability to cluster larger datasets (>1000).

Description of changes

Restore the ability to cluster without opt. ordering to allow clustering on larger datasets.

Includes

• Code changes
• Tests
• Documentation

[ajdapretnar]

This works well for me, but pylint seems to be unhappy. Could we pacify it?

[codecov]

Codecov Report

Merging #4290 into master will increase coverage by 0.05%.
The diff coverage is 94.35%.

@@            Coverage Diff             @@
##           master    #4290      +/-   ##
==========================================
+ Coverage   86.77%   86.82%   +0.05%     
==========================================
  Files         396      396              
  Lines       71550    71792     +242     
==========================================
+ Hits        62089    62336     +247     
+ Misses       9461     9456       -5

[Hrovatin]

Seems to work fine.
However, when a Clustering option is selected it still shows None until the clustering has finished (so a few seconds). Maybe it could show Clustering upon selection (and colour it grey, as it is done now, while it is working). - So that it is immediately evident that the choice was successful. Same goes for None when clustering is removed.