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/multiphaseInterFoam/multiphaseMixture/multiphaseMixture.C

@@ -548,7 +548,8 @@ Foam::multiphaseMixture::nearInterface() const
 
     forAllConstIter(PtrDictionary<phase>, phases_, iter)
     {
-        tnearInt.ref() = max(tnearInt(), pos(iter() - 0.01)*pos(0.99 - iter()));
+        tnearInt.ref() =
+            max(tnearInt(), pos0(iter() - 0.01)*pos0(0.99 - iter()));
     }
 
     return tnearInt;