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.

applications/solvers/multiphase/VoF/setRDeltaT.H

@@ -73,8 +73,8 @@
         rDeltaT.ref() = max
         (
             rDeltaT(),
-            pos(alpha1Bar() - alphaSpreadMin)
-           *pos(alphaSpreadMax - alpha1Bar())
+            pos0(alpha1Bar() - alphaSpreadMin)
+           *pos0(alphaSpreadMax - alpha1Bar())
            *fvc::surfaceSum(mag(phi))()()
            /((2*maxAlphaCo)*mesh.V())
         );