sparse encoding

[Lukas Heumos]

Description of feature

I looked a bit into sparse encoding. One-hot encoding being the most important:

1. scikit-learn's one-hot encoding supports a sparse_output parameter that should return a CSR matrix.
2. We're getting original_values as numpy arrays when calling the function. May or may not be fine.
3. Currently we default the sparse_output parameter to False without checking the type of matrix.
4. The _update_encoded_data does not take sparse matrices into account