EHN RUS/ROS/make_imbalance accepts dataframe in/out

[glemaitre]

RandomUnderSampling, RandomOverSampling, make_imbalance` accepts Pandas DataFrame in and will output Pandas DataFrame.

[codecov]

Codecov Report

Merging #636 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #636      +/-   ##
==========================================
+ Coverage   97.93%   97.94%   +0.01%     
==========================================
  Files          83       83              
  Lines        4784     4810      +26     
==========================================
+ Hits         4685     4711      +26     
  Misses         99       99

Impacted Files	Coverage Δ
imblearn/datasets/_imbalance.py	86.66% <ø> (-1.57%)	⬇️
imblearn/datasets/tests/test_imbalance.py	100% <100%> (ø)	⬆️
...otype_selection/tests/test_random_under_sampler.py	100% <100%> (ø)	⬆️
...rn/over_sampling/tests/test_random_over_sampler.py	100% <100%> (ø)	⬆️
...ling/_prototype_selection/_random_under_sampler.py	100% <100%> (ø)	⬆️
imblearn/over_sampling/_random_over_sampler.py	100% <100%> (ø)	⬆️

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