Allow encoders to be manually defined (for ground stations)

[Capkirk123]

In a similar vein to #41, allow encoders to be manually defined for ground stations. Although the current system is reasonable for power-limited deep space applications, being able to override that encoder choice for Skopos stations would be useful, since commsats are generally bandwidth limited instead.

Most commsats used un-encoded analog FM radio until the late 1970s, and then switched to PSK with 3/4ths or 7/8ths rate encoders. Although RA doesn't have proper support for analog signals, being able to force anything talking to a Skopos ground station to use commsat-like encoding would be useful (ideally defined per-antenna)