Incompatible changes:

- PDB.PDBConfiguration stores atom positions and temperature factors
  (isotropic and anisotropic) in internal units (nm, nm**2) rather than
  in PDB units (Ang, Ang**2).

  Scripts that access these parameters directly must be updated!

New features:

- New module PDBMoleculeFactory permits to work with molecule objects
  that represent exactly the contents of a PDB file.

Improvements:

- The universe method contiguousObjectOffset behaves more reasonably
  for Collection arguments. It doesn't try to make the whole Collection
  contiguous, which for large Collections is impossible anyway.

- New method Universe.configurationDifference.

- PDB.PDBConfiguration: new attributes basis and reciprocal_basis
  describe the crystallographic unit cell and the reciprocal lattice
  cell.

- New method ParticleTensor.trace.

- Additional pressure units kbar, MPa and GPa in module Units.

Bug fixes:

- Hierarchical molecule definitions (using groups) in a MoleculeFactory
  lacked the bonds from the subgroups.

- HarmonicForceField now works correctly with periodic boundary conditions.

- NumPy compatibility fixes.