Rationalize the "pos" function

"pos" now returns 1 if the argument is greater than 0, otherwise it returns 0.
This is consistent with the common mathematical definition of the "pos" function:

https://en.wikipedia.org/wiki/Sign_(mathematics)

However the previous implementation in which 1 was also returned for a 0
argument is useful in many situations so the "pos0" has been added which returns
1 if the argument is greater or equal to 0.  Additionally the "neg0" has been
added which returns 1 if if the argument is less than or equal to 0.

src/engine/engineMesh/fvMotionSolverEngineMesh/fvMotionSolverEngineMesh.C

@@ -81,7 +81,7 @@ void Foam::fvMotionSolverEngineMesh::move()
         );
 
         motionSolver_.pointMotionU().boundaryFieldRef()[linerIndex_] ==
-            pistonSpeed*pos(deckHeight_.value() - linerPoints)
+            pistonSpeed*pos0(deckHeight_.value() - linerPoints)
            *(deckHeight_.value() - linerPoints)
            /(deckHeight_.value() - pistonPlusLayers);
     }