Use low sample rate for SSB/CW demod.

Despite now needing a resampler stage afterward, this does seem to
reduce CPU use.
Add comments to magic numbers in SSBDemodulator.
Introduce "signal types" for sources.

The SignalType object incorporates and replaces the sample rate value
previously provided by sources.

This is planned to be usable to acknowledge audio sources that produce
non-complex output and make further distinctions like "SSB audio" vs.
"FM audio" (where the latter does not correspond to any segment of the
RF spectrum), but does not currently have any functional benefit.