#5: Making constructors of ImageRepresentationXYZ protected - new image representations are to be created via ImageRepresentationXYZ::create().
    This makes much more sense, since the pointer of the newly create image representation is owned by the parent ImageData instead of the caller.

git-svn-id: https://camplinux.in.tum.de/svn/campvis/trunk@443 bb408c1c-ae56-11e1-83d9-df6b3e0c105e