Add public methods to return parameters of Exponential Smoothing models

[amoralico]

There are no public methods to get the parameters of an Exponential Smoothing model.
For example, getAlpha, getBeta, getLevel, etc.
This parameter values are only visible using toString() method of the model.

[pavolloffay]

Thanks for reporting, I'm planning to do more work on this library in the next couple of weeks.

[amoralico]

Thanks to you. The work you've done is awesome.