Utilize PlanarCommand interface

hinerm has hatched a PlanarCommand interface that allows us to simplify development of plugins that work a plane at a time (for 1 plane or all). We should use this in some of our plugins (like the various math ops plugins).

In the math ops plugins there are a couple base classes. And they constrain things to work one plane or all and within the roi or within whole plane via PointSets. We may need to create an adapter class between intervals and PointSets. Or maybe avoid the existing ImageAssignment code in favor of a cursor approach (which could lose parallelism). Or use the new point set implementations in the roi-experiments branch. (Especially the BoundConditionalPointset with the UVInsideRoiCondition when necessary)

Migrated-From: http://trac.imagej.net/ticket/1884

Start by reading the PlanarCommand interface and the existing math ops plugin base classes. Compare their one-plane/all-plane and ROI/whole-plane handling with ImageAssignment, PointSets, and the possible roi-experiments implementations such as BoundConditionalPointset and UVInsideRoiCondition. Done means the relevant math ops plugins use PlanarCommand without losing the required ROI and plane behavior.

Written by the indexing model from the issue text.