RX-888 MKII as uncalibrated solar radio spectrometer

[jcfitzpatrick12]

Once we've provided initial support for the RX888 MKII under issue-218, we'll add a new mode called fft_spectrometer for nodes in the Spectregrams network.

• Spectrograms produced should be compatible with those from the e-Callisto network (see issue-204)
• Recordings are timestamped with GPS (using something like this)

We should consult this repo as a reference example.