Rationalise cameraconfig.json for realsense cameras

[jackjansen]

We really need to clean up cameraconfig.json and the underlying data structures for the realsense capturer.

This is needed because otherwise I'll go mad trying to fix #127 #126 #125 #118

This may mean that cameraconfig.xml for Realsense will no longer be supported, but that isn't too disruptive (realsense was not used in cwipc-sxr).

It may also mean that the realsense-offline capturer (not the newer realsense-playback capturer) has to go.