Change multicast settings

Here’s a question from @turowicz on github:

Is it possible to use aravis (genicam api) to set the camera’s Multicast to Enabled?

The Genicam support in Aravis is, if not complete, wide enough to allow setting any feature supported by your camera. If not, please open an issue.

On the other hand, the stream reception part does not support yet the possibility to only receive the stream, which means only the controller can receive a video stream.

But a pull request implementing this possibility would be gladly accepted.