Elliptic (Cauer) filter order selection.
Return the order of the lowest order digital or analog elliptic filter that loses no more than gpass dB in the passband and has at least gstop dB attenuation in the stopband.
Parameters :  wp, ws : float
gpass : float
gstop : float
analog : bool, optional


Returns :  ord : int
wn : ndarray or float
