Make median agg primitives ignore nans by default

[kmax12]

This makes it consistent with other other aggregation primitives like Sum, Mean, and Std.

Note: I added a column to mock dataset in order to write test. I had to update a 2 other tests as a result.

[codecov-io]

Codecov Report

Merging #61 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #61      +/-   ##
=========================================
+ Coverage   87.19%   87.2%   +0.01%     
=========================================
  Files          74      74              
  Lines        6973    6980       +7     
=========================================
+ Hits         6080    6087       +7     
  Misses        893     893

Impacted Files	Coverage Δ
tests/entityset_tests/test_pandas_es.py	99.73% <0%> (ø)	⬆️
tests/feature_function_tests/test_agg_feats.py	98.51% <0%> (+0.03%)	⬆️
tests/testing_utils/mock_ds.py	94.59% <0%> (+0.04%)	⬆️

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