MinMaxFilter: refactor for N-D

The MinMaxFilter was not properly N-Dimensional, and was using faulty
logic for RGB/interleaved images (the concept of which is now more
generalized, post N-D model).

The MinMaxFilter now tracks:
* single min/max values per plane/image
* min/max values for each planar axis, per axis index/plane/image

Closes #112.