Basic img_proc module revision and re-org using API option

Reorganized the thresholding, histogram, morphology and filter operations so that all direct imports are removed and now loaded via the api.

Remaining tools to follow.
This needs a better name and description.  Note that the `density` documentation is 7 lines long so this clearly needs more explanation ;).

As this is binning into bins which are all width 1 there is no difference between the sum and the integral.