Implement a Model Class and Exponential Smoothing

[hasanemirakin]

Implement a Model superrclass that will be the parent of the forecasting models and create a simple exponential smoothing class. Implement .fit() and .predict() methods for exponential smoothing.

[hasanemirakin]

Inspiration: -https://github.com/statsmodels/statsmodels/blob/main/statsmodels/tsa/base/tsa_model.py (for base model class). No need to implement fully, just a placeholder that we can expand later on is fine. -https://otexts.com/fpp2/ses.html (simple exponential smoothing)

[hasanemirakin]

@muguryalcin has implemented a base model class.

[hasanemirakin]

@muguryalcin has also implemented simple exponential smoothing under smoothing.py