- clearer coding intent. Mark operator() as 'deprecated'

- add bounds checking to get(label) and set(label) methods.

  This gives failsafe behaviour for get() that is symmetric with
  HashPtrTable, autoPtr etc and aligns the set(label) methods
  for UPtrList, PtrList and PtrDynList.

- use top-level PtrList::clone() instead of cloning individual elements

ENH: support HashPtrTable set with refPtr/tmp (flexibility)