Fix a bug with frequency domain filtering

Created by: asouthgate

Fix a bug where frequency domain filtering was not returning an expected result because analog scipy methods were being used despite default analog=False args.

Now, analog and digital filters use corresponding scipy functions (freqs_zpk and freqz_zpk) that take into account fs.

Also, two tests were added, and some test assertions, to check results of frequency domain filtering for some simple examples.

In a few places, tests have changed.