Adds functionality to compute the response of the filter's transfer function at
a given frequency. This uses the Z domain transform. A new test program
zplot_filter (which is compatible with the existing plot_filter) is provided.
Compute the DC gain of the filter (assuming 0 if unstable) and use that to
set the initial conditions of a filter as follows:
x(t<0)=x(0)
y(t<0)=x(0) × dc_gain