No EntitySet required in loading/saving features

[bschreck]

By creating an EntitySet.metadata object, this removes the need to remove (and thus place back upon loading) the references to the EntitySet inside of a list of features before saving to disk. This also eliminates the need for any fanciness before/after pickling.

This also means that calculate_feature_matrix needs an explicit EntitySet passed in, rather than being able to access the referenced EntitySet from the feature objects.

The new API is like this:

ft.save_features(fl, "fl.p")
fl = ft.load_features("fl.p")
ft.calculate_feature_matrix(fl, entityset, **kwargs)

[codecov-io]

Codecov Report

Merging #141 into master will decrease coverage by 0.83%.
The diff coverage is 90.86%.

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
- Coverage   92.21%   91.37%   -0.84%     
==========================================
  Files          72       72              
  Lines        7707     7722      +15     
==========================================
- Hits         7107     7056      -51     
- Misses        600      666      +66

Impacted Files	Coverage Δ
.../feature_function_tests/test_transform_features.py	98.84% <ø> (-0.01%)	⬇️
featuretools/synthesis/dfs.py	100% <ø> (ø)	⬆️
...ols/tests/feature_function_tests/test_agg_feats.py	98.5% <ø> (-0.01%)	⬇️
featuretools/entityset/relationship.py	82.6% <ø> (ø)	⬆️
featuretools/variable_types/variable.py	91.66% <ø> (-0.7%)	⬇️
...aturetools/tests/entityset_tests/test_pandas_es.py	99.75% <ø> (-0.02%)	⬇️
featuretools/primitives/transform_primitive.py	86.29% <0%> (-11.49%)	⬇️
...utational_backend/test_calculate_feature_matrix.py	99.55% <100%> (ø)	⬆️
featuretools/synthesis/deep_feature_synthesis.py	92.6% <100%> (ø)	⬆️
...computational_backends/calculate_feature_matrix.py	98.2% <100%> (-0.39%)	⬇️
... and 13 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dfc43a5...33a853a. Read the comment docs.

[kmax12]

remove this todo

[kmax12]

can we add a docstring to the first line of this so it shows up properly in the documentation?

[kmax12]

add docstring