ENH: added parameter to use periodic x-coordinates

@jaimefrio an error should be raised only for `period==0`... and do `period = abs(period)` to guarantee a positive period.... otherwise I would have to change this:

            xp = np.concatenate((xp[-1:]-abs(period), xp, xp[0:1]+abs(period)))

i.e. adding two times `abs()`