option to disable agc and gains for programs that can not cooperate with the big stream dropouts

[jazzkutya]

Setting AGC on/off causes the hardware stream to stop for a considerable time. Setting gains also stops the stream for a smaller time. Some programs can not handle this.
We should add an option named disablegains to Device::Make keywords to disable gain stuff.
@guruofquality do you agree? Do we get this new option in the SoapyAirspyHF::SoapyAirspyHF constructor?
Will provide a PR