Move 2pi scaling of zpk filters for better encapsulation and more explicit use

Created by: asouthgate

2pi scaling happens by default inside bilinear_zpk for Hz -> rad/s. Extract that functionality so that i) bilinear transform is not also responsible for scaling (single responsibility principle) ii) it is used more explicitly where it is needed iii) the functions are more easily tested.

This MR will be followed by another MR that includes a comprehensive set of tests for filter design functionality, including zpk_transform, as well as bugfixes.