Ticket #1119 (closed defect: fixed)

In implementing code I ran across the following problem with axis validation. If you create a 3 channel unsigned byte dataset but only specify two axes (X & Y) it can display poorly. Once you've created the dataset set it's merged flag true. Code that displays it may show it as grayscale. The last axis (which we want to be channel) is recorded as UNKNOWN. Maybe ImgPlus::validateAxes() should accept null or num axes == num dims. Or complain more loudly. Otherwise its a hard to debug error. (One could define a merged color dataset creator that would avoid these problems but plugin developers might run into the same problem).