Integrate filter designer from libad9361

[tfcollins]

Example filter generation: https://github.com/analogdevicesinc/libad9361-iio/blob/filter-designer-standalone/test/filter_designer_test.c

This will supplement and/or replace the current baseband_auto_rate function which uses a few pre-designed filters. However, extra parameters will need to be exposed in OSC for this designer.

[mhennerich]

Did we come to an conclusion which parameters we must expose?

[tfcollins]

I think exposing: Apass Astop Fpass Fstop

would work. The remaining parameters can be calculated or pulled from existing parameters in the tool.

[rgetz]

Did we validate things where Fpass == Fstop == Fs/2? or Fpass = 90% of Fs/2 and Fstop == Fs/2?