[ENH] add OWRankSurvivalFeatures widget

[JakaKokosar]

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #16 (c46252d) into master (7b6df67) will decrease coverage by 29.28%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master      #16       +/-   ##
===========================================
- Coverage   93.82%   64.54%   -29.29%     
===========================================
  Files           2        3        +1     
  Lines         421      612      +191     
===========================================
  Hits          395      395               
- Misses         26      217      +191     

Impacted Files	Coverage Δ
...urvival_analysis/widgets/owranksurvivalfeatures.py	0.00% <0.00%> (ø)

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