ENH: use Zero when zero-initializing types

- makes the intent clearer and avoids the need for additional
  constructor casting. Eg,

      labelList(10, Zero)    vs.  labelList(10, 0)
      scalarField(10, Zero)  vs.  scalarField(10, scalar(0))
      vectorField(10, Zero)  vs.  vectorField(10, vector::zero)

applications/test/PatchFunction1/Test-PatchFunction1.C

@@ -94,7 +94,7 @@ int main(int argc, char *argv[])
             false
         ),
         mesh,
-        dimensionedVector(vector::zero)
+        dimensionedVector(Zero)
     );
 
     fld.boundaryFieldRef()[0] == function1().value(x0);