Slightly simpler single pole low/highpass (proof)
Avoid lowpass.z and highpass.z R denominator == 0
lowpass and highpass docs using format_docstring
Low/highpass code displacement; R refactor (*_exp)
New lowpass.z_exp and highpass.z_exp strategies
Proof for low/highpass matched Z *_exp strategies
Some updates (+fixes) to all lowpass/highpass docs