[FIX][ENH] Scatter Plot VizRank: some fixes and regard to color

[jerneju]

Issue

Fixes #2037

1. Scatter Plot VizRank uses jittered data which is wrong.
2. Uses custom code for calculating combinations.
3. Uses indices.
4. Informative projections are ranked based on the class variable.

Description of changes

1. Scaled data instead of jittered data.
   2) Uses combinations from itertools.
   3) Now it uses variables instead of indices.
2. Informative projections are ranked based on the selected color variable.

Also added one test to increase coverage (test method row_for_state). Sadly code coverage is not measured when running a code in a thread and that is why test calls this specific method.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2787 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2787      +/-   ##
=========================================
+ Coverage    76.1%   76.1%   +<.01%     
=========================================
  Files         338     338              
  Lines       59760   59767       +7     
=========================================
+ Hits        45479   45486       +7     
  Misses      14281   14281