Improvements:

- Better support of pickle and cPickle. Until now, MMTK's specialized pickler
  had to be used to pickle MMTK's chemical objects. From this version, the
  standard pickler and unpickler can be used as well, although MMTK's
  pickler is more efficient in terms of pickle size and memory use of
  unpickled objects.

- AtomReference objects implement the rich comparison protocol.

Bug fixes:

- The use of force field terms implemented in Python with an integrator
  or minimizer crashed Python on some platforms.

- Modifying a universe after initializing its velocities could lead
  to an infinite recursion.

- Reading a ParticleTrajectory for atoms outside of the trajectory's
  universe now raises an exception.

- Fixed PDB names of oxygens in phosphate groups of DNA

- Added missing database file MMTK/Database/Groups/lysine_neutral_noh