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/combustionModels/diffusionMulticomponent/diffusionMulticomponent.C

@@ -158,7 +158,7 @@ diffusionMulticomponent
     YfStream_(reactions_.size(), 1.0),
     sigma_(reactions_.size(), 0.02),
     oxidantRes_(this->coeffs().lookup("oxidantRes")),
-    ftCorr_(reactions_.size(), 0.0),
+    ftCorr_(reactions_.size(), Zero),
     alpha_(1),
     laminarIgn_(false)
 {