Increase CW filter bandwidth from 200 to 1000 Hz.

The old filter was, in my opinion, too hard to tune sufficiently
accurately (though this really should be fixed by having a zoomed-FFT),
and made the background noise too single-pitched.
Update receiver input frequencies correctly when source is changed.

Also added a regression test, and added to SimulatedSource a center
frequency parameter so as to use it in the test.