Alter warnings for non-uint parcellation images
Alter warnings for non-uint parcellation images

Previously, the user would be warned if their parcellation image was not an unsigned integer. This was perhaps too draconian, as many image formats do not support unsigned integer types.
Instead, such images will now be checked explicitly, and the message reported at CONSOLE level rather than WARN.
I had actually already written code for performing this check, but had commented it out, since moving the IO handler from the Header to the test Image class would result in destruction of the IO class once the test was complete; now, the image is opened for testing independently of the input Header (which should remain const, and retain its existing IO handler).

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3