New features

• JSON model import and export (#469)

Improvements

• Models stored in HDF5 files using SimulationResult.save() now use JSON, rather than pickle
• Raise a ValueError if a rule defined with reverse rate, but expression is not reversible (#471)

Bug fixes

• Fixes for importing certain BNGL and SBML models which previously failed to import (#470)
• Fix SimulationResult.load() for HDF5 files saved with PySB <1.10, which gave a pickle error when loading on PySB 1.10. Such HDF5 files should be re-saved in order to use the new JSON model format, which should be more durable across PySB versions.