Improved filtering definitions and parsing
This PR improves the way FIR and IIR filters are passed to filtering functions, and normalises duplicated code into the relevant modules. Summary is:
- new common methods under
signal.filter_design
for parsing filter arguments, and converting analog ZPK to digital, used by<Array>.filter
instance methods - added common frequency-domain methods module as
frequencyseries._fdcommon
, which now feedsFrequencySeries.filter
andTimeSeries.filter
- improved documentation of methods and functions