Decaylanguage

* Dict-like representation of particle decay chains improved.
* Documentation added to README.

* Parsing of decay files (aka .dec files):
- New Belle II decay models added.
* README updated to provide basic coverage of recent new features.
* Clean-up of obsolete files.

* Universal representation of decay chains:
- Classes ``DecayChain`` and ``DecayMode`` enhanced.
- Tests for class ``DecayChain`` added.
* Parsing of decay files (aka .dec files):
- ``DecFileParser`` class extended.

* Universal representation of decay chains:
- Class ``DecayChain`` introduced.
- Classes ``DaughtersDict`` and ``DecayMode`` enhanced.
* Changes in API:
- ``DecFileParser.build_decay_chain()`` renamed to ``DecFileParser.build_decay_chains()``.
* Package dependent on ``Particle`` package version 0.6.

* Package dependent on ``Particle`` version 0.6.0.
Otherwise identical to version 0.3.2.

* Parsing of decay files (aka .dec files):
- ``DecFileParser`` class extended to understand JETSET definitions.
* Visualisation of decay chains:
- ``DecayChainViewer`` class simplified and improved.
- Decay chain DOT graphs now display HTML particle names.
- More tests.