[FIX][RFC] KMeans upgrade sparse support

[robertcv]

Issue

Fixes #2913
Additionally, I noticed that you can not predict sparse data.

Description of changes

Replaced len(X) with X.shape[0].

Includes

• Code changes
• Tests
• Documentation

[ajdapretnar]

Would you mind adding a nice informative error when the user connects pure Corpus into k-Means (without bag of words)? Now the error is useless. It should say something like: "Data has no attributes." or "Data is missing features."

[robertcv]

@ajdapretnar I can add it but this pull request is probably not the right place for it.

[ajdapretnar]

Ok, let's have it in a different PR then. As far as I am concerned, this PR works.

[ajdapretnar]

Would you mind making pylint happy, so the tests will pass? :) 🙈

third party import "import numpy as np" should be placed before "import Orange"

[codecov-io]

Codecov Report

Merging #3140 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3140   +/-   ##
=======================================
  Coverage   82.33%   82.33%           
=======================================
  Files         335      335           
  Lines       58180    58180           
=======================================
  Hits        47901    47901           
  Misses      10279    10279