Improvements to internal parse_filter method

This PR reworks gwpy.signal.filter_design.parse_filter (renamed from parse_digital_lti) to make it simpler. The return is now just the filter type ('ba' or 'zpk') and the filter components. The main improvement here is that FIR filters aren't shoe-horned through scipy.signal.lti so we don't get annoying warnings about bad filter coefficients.

A small set of other improvements are included, but have no impact on the API.