Preprocessor displaying bug

[patrickleonardy]

Bug Report

Bug: When displaying the preprocessor one gets:
AttributeError: 'PreProcessor' object has no attribute 'categorical_data_processor'

Description

This is caused due to the fact that the PreProcessor object inherits form scikit-learn's BaseEstimator.
In the BaseEstimator docstring is written:

"""Base class for all estimators in scikit-learn.

Notes
-----
All estimators should specify all the parameters that can be set
at the class level in their ``__init__`` as explicit keyword
arguments (no ``*args`` or ``**kwargs``).
"""

In your implementation we have different names in __init__ and for the class attributes.
categorical_data_processor in __init__ and _categorical_data_processor as attribute.

Steps to Reproduce

1. Import PreProcessor
   from cobra.preprocessing import PreProcessor
2. Create PreProcessor object
   preprocessor = PreProcessor.from_params(model_type="regression")
3. Try to show it (in a notebook)
   preprocessor

Expected Result

I was expecting:

Actual Results

I actually got an AttributeError: 'PreProcessor' object has no attribute 'categorical_data_processor'