Set initial condition vector for filter function
The vector zf has the same values that would be obtained
from function filter given past inputs x and outputs y

The vectors x and y contain the most recent inputs and outputs
respectively, with the newest values first:

x = [x(-1) x(-2) ... x(-nb)], nb = length(b)-1
y = [y(-1) y(-2) ... y(-na)], na = length(a)-a

If length(x)