Modify `set_antennaheading` API for parameters to be optional

Right now, `heading` and `width` are both required.  The caller shouldn't have to provide the beam width every time.  Make both parameters optional, and if not provided, don't change that parameter.

Only question: What if neither parameter is provided? Usually this resets to defaults, but I don't know if "antenna heading" really has a default.  Maybe, just turn it off?