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.

etc/codeTemplates/BC/BC.C

@@ -207,7 +207,7 @@ void Foam::CLASS::updateCoeffs()
         (
             "phi"
         );
-    this->valueFraction() = 1.0 - pos(phip);
+    this->valueFraction() = 1.0 - pos0(phip);
 
     PARENT::updateCoeffs();
 }