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

src/meshTools/triSurface/booleanOps/surfaceIntersection/surfaceIntersection.C

@@ -31,7 +31,6 @@ Description
 #include "labelPairLookup.H"
 #include "OFstream.H"
 #include "HashSet.H"
-#include "labelHashSet.H"
 #include "triSurface.H"
 #include "pointIndexHit.H"
 #include "octreeDataTriSurface.H"