WIP Add the first draft of neural prophet

[pavelkrizek]

The first implementation of the neural prophet:

• support for external regressors (if an additional regressor is in X, but not in the self.extra_regressor it will return ValueError) is it OK?
• I had to violate the API a bit - changed static method _transform_data_to_tsmodel_input_format for an instance method. The reason is that unlike in prophet the helper functions for preprocessing the data frame likemake_future_dataframe are not just simple helpers but hides a huge complexity around data preprocessing and I couldn't get it working without them.
• holidays windows features from HolidaTransformer are not passed yet
• holidays are implemented as events with the method add_events, because the holidays' interface is not very flexible and there is no obvious way how to pass customer holidays, events should work the same.