Return (z,p,k) for analog prototype of an Nth order Bessel filter.
The filter is normalized such that the filter asymptotes are the same as a Butterworth filter of the same order with an angular (e.g. rad/s) cutoff frequency of 1.
N : int
The order of the Bessel filter to return zeros, poles and gain for. Values in the range 0-25 are supported.
z : ndarray
Zeros. Is always an empty array.
p : ndarray
k : scalar
Gain. Always 1.