CameraBBox: Fix bug
CameraBBox: Use initial guess if available
CameraBBbox: Improve the search for initial guess

Also introduce the variable cambbox_mode. When computing the DEM camera bbox
we prefer to preserve established behavior.
ip: Add parallel describe_interest_points

Normally this isn't an intensive process. However with large enough
images this actually matters.

I just cloned the api design from detect_interest_points.