Ml-tooling

- Bug fix for logging when feature unions (DFFeatureUnion) had tuples

- Hot fix python version to 3.7

- Breaking change - Model methods load_estimator and save_estimator now takes a Storage class that defines how and where to store estimators.
- Added the ability to declare that a saved model should be a production estimator.
- Added corresponding `.load_production_estimator` to `Model`

- Removed gitpython as a dependency

- Replaced custom feature permutation importance with sklearns implementation from v0.22
- Breaking change - Dataset is now a separate object that has to be instantiated outside Modeldata
- Breaking change - ModelData is now renamed to Model
- Added new properties `is_estimator` and `is_regressor` which checks what type of estimator is used

- Joblib is now a dependency, instead of being vendored with scikit-learn
- Updated requirements