FreeDV support
Added FreeDV support in the form of a modulator and a demodulator plugin
Limitations:
- Limited distribution as a binary (see next).
- No data support. This will come later and may replace the pretty useless CW keyer.
- No server plugins. This will come in the next update
- No support for 700D mode. This is disabled in the GUI only actually. Despite all my efforts I cannot get it to work reliably other modes work just fine in the same conditions.
In order to get Codec2/FreeDV support when building from source check the updated Wiki page: https://github.com/f4exb/sdrangel/wiki/Compile-from-source-in-Linux#codec2freedv