STYLE: revert bitSet/HashSet setMany() to set() method (issue #837)

- improves backward compatibility and more naming consistency.
  Retain setMany(iter1, iter2) to avoid ambiguity with the
  PackedList::set(index, value) method.

src/parallel/decompose/decompositionMethods/structuredDecomp/structuredDecomp.C

@@ -85,7 +85,7 @@ Foam::labelList Foam::structuredDecomp::decompose
     labelHashSet patchCells(2*nFaces);
     for (const label patchi : patchIDs)
     {
-        patchCells.insertMany(pbm[patchi].faceCells());
+        patchCells.insert(pbm[patchi].faceCells());
     }
 
     // Subset the layer of cells next to the patch