Fix NOAA APT waterfall bandwidth

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 (closed)