[ENH] OWFeatureConstructor: Add new functions from numpy

[lopezco]

Issue

Implements new functions

Description of changes

• Add numpy 'nan' functions as replacement for min, max, sum
• Add other functions: mean, std, var...

Includes

• Code changes
• Tests
• Documentation

[thocevar]

Overriding sum with nansum is counterintuitive. In case of an unkown value the sum should be unkown as well. Consider adding nansum and others as separate functions.

Please rebase the branch to resolve conflicts and fix the pylint errors reported by Travis CI.

[codecov-io]

Codecov Report

Merging #2410 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2410      +/-   ##
==========================================
- Coverage   75.11%   75.07%   -0.04%     
==========================================
  Files         323      323              
  Lines       56902    56903       +1     
==========================================
- Hits        42743    42722      -21     
- Misses      14159    14181      +22

[thocevar]

There was a problem with your rebase. Most likely you rebased on a branch that was not synced with biolab:master and ended up with 100+ commits in the pull request. I'll handle the merge from here on, thank you for your contribution.

[lopezco]

wow! sorry @thocevar you're right. I didn't check that