Augment fourier - dataframes only

[GTimothee]

Hi,
Let me know your thoughts about this implementation.
I think it is what was expected. With order 1 (k=1) I got the same results than the current function.
I only implemented the function for the DataFrames, and if it is validated it will be straightforward to do the same for the groups.
I would also like to replace those for loops, but for the moment I was more interested in the correctness of the results :)

fix #22
NB this is a working version. I implemented my fix in a _v2 function so that the other function still works :)

[mdancho84]

OK, I've just made a few updates to help make it more consistent.

max periods was changed to periods.
periods: Union[int, Tuple[int, int], List[int]] = 1 which is consistent with how augment_lags works.

I think it looks pretty good. See if you can do the grouped version.

[mdancho84]

Also, make sure to do a git pull. I've relocated to the feature_engineering submodule.