COMP: Fix minor warnings generated with gcc8
- Catch by value instead of reference.
- Multi-line comment without /* */ notation.
[77/796] Building CXX object Libs/MRML/Core/CM...LCore.dir/vtkMRMLSegmentationStorageNode.cxx.o
/Software/Slicer/Slicer-src/Libs/MRML/Logic/vtkMRMLSliceLogic.cxx:85:5: warning: multi-line comment [-Wcomment]
// foreground \
^
/Software/Slicer/Slicer-src/Libs/MRML/Logic/vtkMRMLSliceLogic.cxx:98:5: warning: multi-line comment [-Wcomment]
// foreground > AddSubForegroundCast \
^
/Software/Slicer/Slicer-src/Libs/MRML/Logic/vtkMRMLSliceLogic.cxx:103:5: warning: multi-line comment [-Wcomment]
// ... AddSubOutputCast > AddSubExtractRGB \
^