Generalize infer_variable_type, convert_variable_data and convert_all_variable_data methods

[bukosabino]

#338

[codecov]

Codecov Report

Merging #423 into master will increase coverage by 0.02%.
The diff coverage is 97.35%.

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
+ Coverage   96.29%   96.32%   +0.02%     
==========================================
  Files          93       95       +2     
  Lines        8455     8514      +59     
==========================================
+ Hits         8142     8201      +59     
  Misses        313      313

Impacted Files	Coverage Δ
...eaturetools/tests/utils_tests/test_entity_utils.py	100% <100%> (ø)
featuretools/entityset/entity.py	95.75% <100%> (+0.1%)	⬆️
featuretools/utils/entity_utils.py	95.78% <95.78%> (ø)

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[kmax12]

some initial feedback. the test cases still need more work before this is ready to merge

[kmax12]

Should be ready to merge after my comments are addressed. Thanks

[kmax12]

just a couple last comments. looking good overall. thank you!

[kmax12]

This ready to go. We will merge it master after the next release of Featuretools goes out, which should be this week.