Add utilities for periodic boundary conditions about derivative

[andgoldschmidt]

E.g. for trend filtered derivatives, potential options are:

1. use pinv for a periodic derivative (this leads to relaxed solution curvature constraints)
2. use padded data (not what a user would expect, and hard to say how much padding to use)
3. enforce periodicity in the final result <--
   Option 3 matches the behavior of periodicity in splines, for example. This is the best option for global methods.

For Savitzky-Golay, padding is the natural choice.

[andgoldschmidt]

Closed with 17cd88d. If general padding utilities are desired (ruled unnecessary for now), open a new issue. Also, if periodic global methods are desired, open a specific issue for each method (leaving this alone for now).