ENH: free/nullify of PtrList and UPtrList

- Allows clearing or freeing pointers without touching the underlying
  list size. Was previously only for PtrDynList, but now available on
  UPtrList, PtrList as well.

- add transfer() method to PtrDynList to avoid potential slicing.

src/sampling/sampledSet/sampledSets/sampledSets.C

@@ -155,7 +155,7 @@ void Foam::sampledSets::gatherAllSets()
 
     const PtrList<sampledSet>& localSets = *this;
 
-    gatheredSets_.clear();
+    gatheredSets_.free();
     gatheredSets_.resize(localSets.size());
     gatheredSorting_.resize_nocopy(localSets.size());
     globalIndices_.resize_nocopy(localSets.size());