Frequency space filter test is likely broken

[angus-g]

For consistency with the analysis module, we should probably use da.fft in the FrequencySpaceFilter. We've already learned that fftfreq expects a sample spacing (rather than frequency), so the fs argument in test_frequency_filter is wrong. We should also be careful about angular vs. linear frequencies (there's probably a factor of 2*pi missing in the test/filter).