Improvements:

- MMTK.Dynamics.Heater now accepts negative temperature gradients.

- More comprehensive test suite.

- New example script Visualization/vector_field_chimera.py

Bug fixes:

- Dipole calculation (method dipole) used to crash.

- Methods boundingBox and boundingSphere used to crash.

- Indexing a subset of a trajectory crashed with NumPy.
  (I hope this is the last NumPy-related bug!)

- Replacing residues in a peptide chain could crash.

- Corrections in the manual.