debug_object_detection_test.py: clear camera buffer as needed

Added a function, clear_camera_buffer(), which retrieves but
does not display frames from the camera. Useful for consuming
the camera's buffer when it has stale data.

Signed-off-by: Scott Garman <scott.a.garman@intel.com>
minnowboard_fish_picker-upper.py: added retry and other improvements

After picking up, the program will check to see if a fish is still
visible by the camera, and if so, will re-center and retry picking
it up.

Other improvements to clear the camera buffer and display live video
during more operations.

Bugfixes for some edge cases.

Signed-off-by: Scott Garman <scott.a.garman@intel.com>