Give pyDARM the flexibility to ingest a model of analog AA filters in a format other than a .mat file

As we update the physical analog AA filter with the upgrades to the OMC DCPD sensing chain, we should take he opportunity to detach ourselves from the clunking loading in of a .mat file by python. While we'll still need backward compatibility, new options should include:

• an hdf5, npz, or just a text file of the model on a pre-defined frequency vector
• defining the parameters of a filter so that we can "just" create it from scratch on an arbitrary frequency vector using python filter creating tools in, say, the signal package.