In-house frequency response function supporting dense/sparse types

Inputs: - SS: instance of StateSpace class, or scipy.signal.StateSpace* - wv: frequency range - dlti: True if discrete-time system is considered.

Outputs: - Yfreq[outputs,inputs,len(wv)]: frequency response over wv

Warnings: - This function may not be very efficient for dense matrices (as A is not reduced to upper Hessenberg form), but can exploit sparsity in the state-space matrices.