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/CompactListList/Test-CompactListList.C

@@ -47,7 +47,7 @@ int main(int argc, char *argv[])
         Info<< "cll1:" << cll1 << endl;
 
         // Resize and assign row by row
-        labelList row0(2, label(0));
+        labelList row0(2, Zero);
         labelList row1(3, label(1));
 
         labelList rowSizes(2);