@staticmethod
def fit_category_nans(df):
    """ONLY FIT: Convert np.NaNs to a category 'NaN'"""
    for column in df.select_dtypes(include=['category']).columns:
        if df[column].isnull().any():
            df[column].cat.add_categories('NaN', inplace=True)
            df[column].fillna('NaN', inplace=True)
