Select low information features

[bschreck]

Removes two functions: selection.select_high_variance_features and selection.select_percent_null(), and replace them with selection.remove_low_information_features().

The new function removes all features that only contain null values, or that only contain a single value.

The reason to remove these functions is that we are unsure about whether it is generally a good idea to remove these types of features. However, it is almost always a good idea to remove features that provide no information.

[codecov-io]

Codecov Report

Merging #18 into v0.1.11 will increase coverage by 0.32%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           v0.1.11      #18      +/-   ##
===========================================
+ Coverage    86.83%   87.16%   +0.32%     
===========================================
  Files           73       73              
  Lines         6768     6700      -68     
===========================================
- Hits          5877     5840      -37     
+ Misses         891      860      -31

Impacted Files	Coverage Δ
featuretools/selection/selection.py	100% <100%> (ø)
featuretools/selection/api.py	100% <100%> (ø)	⬆️
featuretools/tests/selection/test_selection.py	100% <100%> (ø)

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[kmax12]

looks good to me