ENH: add get() retrieval of a pointer from PtrLists, HashPtrTable

- naming similarity with autoPtr, unique_ptr and other containers.

  For UPtrList derivatives, this is equivalent to the existing
  operator(). The read-only variant is also equivalent to the
  single-parameter 'set(label)' method.

  With PtrList<T> list(...) :

      const T* ptr = list.get(10);
      if (ptr)
      {
          ptr->method();
      }

  vs.
      if (list.set(10))
      {
          list[10].method();
      }

  For HashPtrTable there is only a read-only variant which is equivalent
  to testing for existence and for value.

  With HashPtrTable<T> hash(...) :

      const T* ptr = list.get("key");
      if (ptr)
      {
          ptr->method();
      }

  vs.
      if (list.found("key"))
      {
          // Fails on null pointer!!
          list["key"].method();
      }

Use of get() is largely a matter of taste or local coding requirements