Decaylanguage

* 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.

* Parsing of decay files (aka .dec files):
- Update to latest LHCb DECAY.DEC file.
* Visualisation of decay chains:
- ``DecayChainViewer`` class made more robust.
- Better tests.
* Miscellaneous:
- Demo notebook updated.
- README updated with latest package functionality.
- Python wheels generation added.
- Zenodo DOI badge added to README.

* Decays modelling:
- Updates to Mint related particle data files.
* Parsing of decay files (aka .dec files):
- Lark parser files added, for ``.dec`` decay files.
- ``DecFileParser`` class introduced, with documentation and test suite.
- Various ``.dec`` test decay files added.
* Visualisation of decay chains:
- ``DecayChainViewer`` class introduced, with documentation and test suite.
* Universal representation of decay chains:
- First "building block" classes ``DaughtersDict`` and ``DecayMode`` introduced,
with documentation and test suite.
* Package dependencies:
- Package made dependent on Scikit-HEP's ``Particle`` package.
- Redundant code removed.
* Continuous integration:
- CI with Azure pipelines introduced.
- CI with Travis and AppVeyor removed.
* Miscellaneous:
- Demo notebook added, with a launcher for Binder.
- Copyright statements added to repository files.
- General clean-up and minor bug fixes.