UDP sink plugin
This is for Linux only. Included in Mac O/S build but not tested.
New sink (Tx) channel plugin to get the channel input from UDP. This is the Tx counterpart of the UDP source plugin.
Main purpose is to add the linear transponder capability. It is also possible to transpose individual channels with various output modulations: AM, FM, SSB. Thus in combination with the UDP source plugin cross modulation transponders are possible (AM->FM, FM->SSB, ...) but also FM->FM, AM->AM, ...
More details here: https://github.com/f4exb/sdrangel/blob/master/plugins/channeltx/udpsink/readme.md
This implements issue #37