Add pole zero mapping

[albheim]

From #441, add pole zero mapping for c2d on zpk system. Maybe need to rethink how to handle c2d a bit more general.

[baggepinnen]

There are some related stuff here

ControlSystems.jl/src/discrete.jl

Line 225 in bde0c05

function c2d_poly2poly(p, Ts)