Hydra alternate approach - two devices sharing an underlying hydra_system

Had trouble with the current design in making left and right hands come from a single device, so I tried a different approach (since this has a fixed, known number of sub-units): the prober produces 2 devices, one for each controller, which share a (reference counted) common hydra_system object.

Needed some changes to the prober to allow this, but it works well. Also adjusted the tracking type enum - I initially tried saying I had pose by not providing the TRACKING_NONE flag, but it turns out that tracking_none is a kind of tracking that isn't ever updated by the driver, vs not having that flag resulting in segfaults.

Probably not for merging yet since I simply "hacked the hack" of binding.

cc @wallbraker