Mean primitive changes

[rwedge]

Fixes #393, changes ignore_nans to skipna, and set skipna=True by default

[codecov]

Codecov Report

Merging #395 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
+ Coverage   95.78%   95.79%   +<.01%     
==========================================
  Files          90       90              
  Lines        7882     7896      +14     
==========================================
+ Hits         7550     7564      +14     
  Misses        332      332

Impacted Files	Coverage Δ
...ools/primitives/standard/aggregation_primitives.py	95.9% <100%> (+0.15%)	⬆️
...aturetools/tests/primitive_tests/test_agg_feats.py	99.2% <100%> (+0.01%)	⬆️

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[kmax12]

LGTM