ADD: multi-label erosion by distance


git-svn-id: https://www.nitrc.org/svn/cmtk/trunk@5259 42a5c34f-2066-0410-bec5-ba365beb4995
ADD: test for multi-label erosion


git-svn-id: https://www.nitrc.org/svn/cmtk/trunk@5260 42a5c34f-2066-0410-bec5-ba365beb4995