The flowgraph for decoding NOAA APT images generates a waterfall with a
bandwidth depending on the default samplerate of the SDR hardware.
USRPB200 defaults to 500 ksps, which produces an output of 125 ksps
after the "first_stage_decimation" which is fixed to a value of 4.
Changing the hardware to an "rtlsdr" also changes the default
samplerate to 1 Msps which results in a waterfall bandwidth of 250 kHz
(+- 125 kHz). Connecting the waterfall sink behind the FFT Filter,
which has a decimation set to match an output samplerate of 125 kHz,
seems to be correct! This produces waterfalls with a bandwidth of
125 kHz (+- 62,5 kHz).
Fixes issue #128