for remove_incoherent_edge_pixels().

to select thresholds.

the odr violation checks don't erroneously trigger.

threshold if none is given.  Also depreciated auto_threshold_image() since
using partition_pixels() to pick the threshold is generally better.

to output from the subprocesses.

conform to dlib's generic image interface.  This makes dealing with
numpy images in pybind11 modules much nicer.