Switch default filter design to use Second Order Sections not ZPK.

Created by: areeda

Previously we combined the ZPKs and applied them all at once. This PR uses SOS implementations and applies them one at a time. The differences are real but fairly subtle.

Here's a comparison using scipy's freqz (which shows no difference) and impulse response functions which show amplitude differences: ldvw group #1

This example compares the two on an actual channel showing that SOS preserves features in highly attenuated frequencies: PEM accelerometer