Ordinal Encoder added to the preprocessing module

[pachecoreina]

OrdinalEncoder added to the preprocessing module

The code that counts the feature:category occurrences is now a function that both OneHotEncoder and OrdinalEncoder use.

The function get_feature_names in utils was refactor to add dtype = None as a parameter so it accepts all types of data.

[MBrouns]

Thanks for the PR! In the test_preprocessing.py file there's a test_standard_checks function that runs a bunch of the default scikit-learn checks on the OneHotEncoder. Could you make a similar function for the OrdinalEncoder?

[pachecoreina]

Thanks for the PR! In the test_preprocessing.py file there's a test_standard_checks function that runs a bunch of the default scikit-learn checks on the OneHotEncoder. Could you make a similar function for the OrdinalEncoder?

Pushed a new commit with the standards checks for the OrdinalEncoder. Also added LabelEncoder and its tests.