BUG: #3561 did not check if index < 0, which causes crashes

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23748 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Add function "removeObserver" to slicer.util.VTKObservationMixin

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23749 3bd1e089-480b-0410-8dfb-8563597acbee