ENH: use objectRegistry/IOobjectList sorted instead of lookupClass

- in most cases a parallel-consistent order is required.
  Even when the order is not important, it will generally require
  fewer allocations to create a UPtrList of entries instead of a
  HashTable or even a wordList.

src/sampling/sampledSet/sampledSets/sampledSets.C

@@ -155,8 +155,7 @@ void Foam::sampledSets::gatherAllSets()
 
     const PtrList<sampledSet>& localSets = *this;
 
-    gatheredSets_.free();
-    gatheredSets_.resize(localSets.size());
+    gatheredSets_.resize_null(localSets.size());
     gatheredSorting_.resize_nocopy(localSets.size());
     globalIndices_.resize_nocopy(localSets.size());