Add trigger_mode parameter to cameras
Added trigger-mode test
Added BufferedCamera
Added BufferedCamera test
Grab returns a frame no matter the trigger mode is

Except the trigger mode is external and the grab times out.
In that case CameraError is raised.
Make use of the new grab in acquire

And also rename to frames.
Wrap GError into the CameraError
Refactor dummy cameras

and complete tests.
Refactor uca camera classes