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/interPhaseChangeFoam/phaseChangeTwoPhaseMixtures/Kunz/Kunz.C

@@ -87,7 +87,7 @@ Foam::phaseChangeTwoPhaseMixtures::Kunz::mDotP() const
     return Pair<tmp<volScalarField>>
     (
         mcCoeff_*sqr(limitedAlpha1)*(1.0 - limitedAlpha1)
-       *pos(p - pSat())/max(p - pSat(), 0.01*pSat()),
+       *pos0(p - pSat())/max(p - pSat(), 0.01*pSat()),
 
         (-mvCoeff_)*limitedAlpha1*neg(p - pSat())
     );

applications/solvers/multiphase/interPhaseChangeFoam/phaseChangeTwoPhaseMixtures/Merkle/Merkle.C

@@ -83,7 +83,7 @@ Foam::phaseChangeTwoPhaseMixtures::Merkle::mDotP() const
 
     return Pair<tmp<volScalarField>>
     (
-        mcCoeff_*(1.0 - limitedAlpha1)*pos(p - pSat()),
+        mcCoeff_*(1.0 - limitedAlpha1)*pos0(p - pSat()),
         (-mvCoeff_)*limitedAlpha1*neg(p - pSat())
     );
 }

applications/solvers/multiphase/interPhaseChangeFoam/phaseChangeTwoPhaseMixtures/SchnerrSauer/SchnerrSauer.C

@@ -136,7 +136,7 @@ Foam::phaseChangeTwoPhaseMixtures::SchnerrSauer::mDotP() const
 
     return Pair<tmp<volScalarField>>
     (
-        Cc_*(1.0 - limitedAlpha1)*pos(p - pSat())*apCoeff,
+        Cc_*(1.0 - limitedAlpha1)*pos0(p - pSat())*apCoeff,
 
         (-Cv_)*(1.0 + alphaNuc() - limitedAlpha1)*neg(p - pSat())*apCoeff
     );