ENH: avoid memory leaks for HashPtrTable, PtrMap insertion (issue #749)

- disallow insert() of raw pointers, since a failed insertion
  (ie, entry already existed) results in an unmanaged pointer.

  Either insert using an autoPtr, or set() with raw pointers or autoPtr.

- IOobjectList::add() now takes an autoPtr instead of an object reference

- IOobjectList::remove() now returns an autoPtr instead of a raw pointer