Generic way to set gains for any SDR hardware

[ha7ilm]

Currently it is hardwired for HackRF and RTL-SDR.

See also: ha7ilm/openwebrx#94

[guruofquality]

I sure that nobody want to parse strings, but if we made the --gain option take a string, we could use the same markup used for device arguments to get a dictionary of gains. We can use an existing call SOAPY_SDR_API SoapySDRKwargs SoapySDRKwargs_fromString(const char *markup); to parse something like -g "AMPL=12, VGA=4" (for example). Its easy to iterate through a SoapySDRKwargs in C and apply each gain in the dictionary.

For the automatic/overall gain use case, --gain=16 will result in a SoapySDRKwargs with one entry with a key set to that string and an empty value -- so in this case we could know to use the setGain() with no named elements.

[guruofquality]

this is the -g option in rx_fm (for example) it parses a gain string for key/value pairs of gain settings