reconsider default values

[targos]

Current default values are not intuitive. The SG function is first a smoothing algorithm and should behave like this if the derivative option is not entered.
See #15

[maasencioh]

@targos , you are right, it's a smoothing algorithm, but the usual application is for derivatives

[maasencioh]

@mljs/collaborators what should we do with this?, I think that the default value should be the first derivative.

[andcastillo]

The default value should be 0. In principle it is intended to smooth the function, but S-G extended it to approach derivatives as well. I don't think there is a preferred derivative order. It will depend on the application.