Converter for Kraken SDR Recordings

[Teque5]

Originally discussed in #23, we want to implement a converter for recordings made by Kraken RF recorders.

• Use CLI sigmf_convert entry point and use sigmf.convert.kraken.kraken_to_sigmf() as entry point.
• Use sigmf.fromfile() entry point and file extension or ???? detection method.
• Support --archive and --ncd modes if possible
• Add example recording to nonsigmf examples repo
• Use example recordings in nonsigmf examples repo in test_convert_kraken.py
• Add to converters.rst in Documentation

Original footwork by @KelseyCreekSoftware:

I do believe it is simple raw IQ data (interleaved 5 channel).

It may be small set of code to convert the meta data saved in the log files and docs on how to take the IQ data and bundle that together. Possibly with a third file - Timestamped GPS data saved as an archive file. The goal would be to create a useful bundle that could be sent to others to dig into the RF content, with as much information as possible. I'm just digging into how that could be done, and I have started a project notes page.

This existing code seems to gather the stream like the IQ data and save it out as SigMF.

It could also make sense to just use the Kraken SDR Source Block and a SigMF block connected together.
GNU Radio Block - https://github.com/krakenrf/krakensdr_docs/wiki/08.-GNU-Radio-Block#how-the-krakensdr-source-block-works

These docs describe the local data recording.

There is also a "heimdall" recorder, for IQ data acquisition.