Frame manipulation

Contains methods that are applying feature engineering artifacts to the prediction dataframe.

• apply_label_encoders
• apply_one_hot_encoding

Also contains these helper methods:

• to_numeric_features_df: extracts the numeric features from the dataframe, and returns them as a new dataframe.
• remove_invalid_values: removes the invalid values from the dataframe.
• remove_datetime_columns: removes all columns of dtype datetime64.
• fill_nan_with_mode: fills the NaN cells in each numeric column with the most frequent value (aka mode) of that column.