opencv3 dependency

[sfalexrog]

As of Melodic, opencv3 is no longer a ROS package and is not listed among known packages. Please consider removing it as a direct dependency and use cv_bridge (or other packages that depend on OpenCV) instead.

I know how inconvenient and painful this is (you'll have to ensure your code builds against OpenCV 3.2 and runs correctly), but if we're switching to Melodic, we'll have to do that. As an alternative we can probably introduce an opencv4 dependency (which will resolve to libopencv-dev=4.1.1), but that would break a lot of stuff and make these packages nearly unusable for anyone not using our Clever platform.