Minor cleanup of adapthist module

Per @tonysu's comments on #893, I cleaned up this module a bit.  

To answer Tony's questions:
- The `NR_OF_GREY` is from the original algorithm, in which they used 2**14.
- There is no reason to require 2 or more regions (tested now with 1), that was leftover from CLAHE.

Also cleaned up the shape twiddling a bit.