Defer top block initialization to Config.
Feature to visualize the flowgraph with graphviz.
Use SignalType to describe demodulator outputs.
Abstract over number of audio channels in Top.
Client support for mono.
Receiver allows mono-output demodulators.
Demodulators are mono where possible.
Implement mono output in Receiver.
Add config option for completely mono audio.
Don't use an unnecessary streams_to_vector block.
Fix broken OsmoSDR correction_ppm setter.
OsmoSDR filter bandwidth setting may be zero.

The value zero is never included in get_bandwidth_range(), but means
"automatic setting", and is sometimes the only value supported.