[FIX] Tree: Sparse Support

[jerneju]

Issue

Fixes #2370 .

Description of changes

Work in progress.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2430 into master will increase coverage by 1.84%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2430      +/-   ##
==========================================
+ Coverage    73.9%   75.74%   +1.84%     
==========================================
  Files         321      321              
  Lines       55993    60239    +4246     
==========================================
+ Hits        41379    45630    +4251     
+ Misses      14614    14609       -5

[nikicc]

@jerneju is this ready for review? IMO this should also go into the next release?

[jerneju]

@nikicc: Yes, it is ready.

[janezd]

Works, but it's extremely slow even on reasonably sized sparse data.

https://github.com/biolab/orange3/pull/2430/files#diff-e09cca17420838b03154933ede608989R171 changes the column to dense. Since this is done column by column, it is (memory-wise) better than changing the whole matrix to dense but it's time consuming. A better solution - but one requiring a bit more programming - would be to implement a separate _tree_scorers.contingency, branches_from_mapping and build_tree (active_inst[branches == br]) to work with sparse data.

@nikicc said we should merge it as it is, but please start working on a better solution.