Impulse response invariant discretization of fractional second order filter.

The function compute a discrete-time transfer function to fractional second order low-pass filter.

irid_fsof function is prepared to compute a discrete-time finite dimensional (z) transfer function to approximate a continuous-time fractional second order low-pass filter (LPF) [1/(s^2 + a*s + b)]^r, where "s" is the Laplace transform variable; "r" is a real number in the range of (0,1); a and b are the time constant of LPF [1/(s^2 + a*s + b)]^r, where a, b >= 0.

The proposed approximation keeps the impulse response "invariant"