Allow observer to set the center frequency when schedules transpoder

Currently when a transponder is scheduled, the frequency that client will observe is the low downlink one. However in order to observe a transmission within the range you will need as observer to have the ability to set the frequency you want as a center frequency. Just to make it clear the set frequency will be (and be validated to be) between low and high downlink frequences.

This issue came from a discussion started in this tweet.