HashSet gets additional operators

- operator+=  : add in the listed keys
 - operator-=  : remove the listed keys
 - operator&=  : intersection of keys
 - added xfer constructor (just in case)
 - moved labelHashSet typedef to HashSet.H, for consistency with the
   wordHashSet typedef being there and since it is used so often

applications/utilities/mesh/manipulation/splitMesh/regionSide.H

@@ -45,7 +45,7 @@ SourceFiles
 #ifndef regionSide_H
 #define regionSide_H
 
-#include "labelHashSet.H"
+#include "HashSet.H"
 #include "typeInfo.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //