ENH: HashTable::emplace_set() method, HashPtrTable support for unique_ptr

- forwarding like the emplace() method, but overwriting existing
  entries as required

- propagate similar changes to HashPtrTable

  For example, with HashPtrTable<labelList> table(...) :

  With 'insert' semantics

      table.emplace("list1", 1000);

  vs
      if (!table.found("list1"))
      {
          table.set("list1", new labelList(1000));
      }
  or
      table.insert("list1", autoPtr<labelList>::New(1000));

  Note that the last example invokes an unnecessary allocation/deletion
  if the insertion is unsuccessful.

  With 'set' semantics:

      table.emplace_set("list1", 15);

  vs
      table.set("list1", new labelList(15));