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)

src/OpenFOAM/db/IOobjects/decomposedBlockData/decomposedBlockData.C

@@ -796,7 +796,7 @@ Foam::label Foam::decomposedBlockData::calcNumProcs
     (
         reinterpret_cast<const char*>(&nSendProcs),
         List<int>(nProcs, sizeof(nSendProcs)),
-        List<int>(nProcs, 0),
+        List<int>(nProcs, Zero),
         reinterpret_cast<char*>(&n),
         sizeof(n),
         comm