Add frequency and gain parameters to flow

It would be great if the program allowed to set the initial values for center frequency and gain, like the https://github.com/vmlemon/AirProbe-GPRS/blob/master/gsm-receiver/src/python/gsm_receive_rtl.py did.

Just to learn, I edited the flow to add them with your default values and my (limited) tests showed no problem but feel free to discard the pull and do it properly.

Thank you!