Need to apply anti-aliasing lowpass filter prior to Lanczos interpolation

[liamtoney]

As a pre-processing step, we resample waveforms using the Lanczos method. We do this here:

rtm/rtm/waveform.py

Line 90 in 6a163db

st_i.interpolate(sampling_rate=decimation_rate, method='lanczos',

And here:

rtm/rtm/waveform.py

Line 96 in 6a163db

st_i.interpolate(sampling_rate=lowest_fs, method='lanczos', a=a_param)

Currently we are not applying an anti-aliasing filter first — this can lead to bad results! We need to add code to apply an appropriate lowpass filter prior to the interpolate() method being called.

[liamtoney]

This is not as bad as I initially thought... interpolating occurs after filtering. So often (but not always) the user-defined bandpass will act as an AA filter. We still should fix!