Fix registry bug at program exit

If a python program terminates, objects are deleted in a random order. If the pshmem package registry is deleted before all MPIShared objects are deleted, then gracefully ignore requests to unregister the already deleted shared memory buffers.

What's Changed

• Prevent errors caused by unregistering memory twice. by @tskisner in #27

Full Changelog: 1.2.0...1.2.1