I've consolidated the yt and memory-optimization branches in the yt repo in mercurial. If you want to use either before this merger, the last changeset hashes before the merger are:
97e2160d876b (memory-optimization) ab99adbbe406 (yt)
The new 'yt' branch now contains the massively parallel halo finder from Stephen Skory, as well as a fully rewritten hierarchy class. This will become the new dev trunk relatively soon. If you are concerned about changes in it, please clone it and test it. There were two bugs I was notified of in the svn trunk after last night's merger that I have now fixed.
If you find a problem in this branch, you can either fix it or add it as a unit test to tests/test_*.py.