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 does not handle rotations.  See https://github.com/tacaswell/leidenfrost/blob/master/leidenfrost/ellipse.py#L66 which will return the edges of an ellipse.