BUG: #2737, #2929 improve RemoveIslands tool

This adds a new "Morphological" island removal algorithm that has
been tested as useful on the brain mask that is output from the
diffusion tensor estimation module.  It may be useful for other
applications as well.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22299 3bd1e089-480b-0410-8dfb-8563597acbee