Tests for pid_design.jl and discrete.jl

[mfalt]

The code in pid_design.jl and discrete.jl currently has 0% and 20% test coverage, we should definitely improve this if we want to support that functionalty.

We should also convert the functions in piddesign/discrete to use the new functions numvec/denvec/numpoly/denpoly where it is useful.

I could help out with this but @baggepinnen is tagged since the code was implemented by him.