RX-888 MKII as uncalibrated solar radio spectrometer

[jcfitzpatrick12]

Add a new operating mode to the rx888mk2 receiver that produces spectrograms compatible with the e-Callisto network. Before implementing, the following needs to be understood:

• File format & layout: what FITS conventions does the e-Callisto network expect?
• Spectrogram relationships: how do spectrograms produced in this mode relate to those produced by CALLISTO receivers, given the differing hardware and means of recording the spectrogram?
• Configs: what parameters should be user-configurable?
• Software compatibility: output files must be readable by existing Callisto tooling.
• Publishable: the resulting FITS files should be publishable to Astrodoncel.

A corresponding mode should be added to the signal_generator receiver for hardware-free testing.

Reference: AlexandroRP99/e-Callisto_Py_RX-888_MK_II