Decaylanguage

* Universal representation of decay chains:
- More documentation and testing.
- Allow default class method `DecayMode.from_pdgids()` mimicking the default constructor `DecayMode()`.
- Fix in logic of ``DecayChain.flatten()``.
- `DecayChainViewer` class adapted to make use of ``graphviz``.
* Dependencies:
- Got rid of dependency on ``pydot``, as the package is no longer maintained.
- Replaced with dependency on ``graphviz``, made a requirement.
* Tests:
- Check for some expected warnings, to get rid of obvious warnings.
* Miscellaneous:
- Follow Scikit-HEP org guidelines for code development and packaging.

* Miscellaneous:
- Code refactored in the CI by Sourcery.ai.
- Updates to versions of pre-commit hooks.

* Parsing of decay files (aka .dec files):
- New decay models as taken from most-recent EvtGen documentation.

* Environment YAML file updated to ``Particle`` version 0.14.

* Dependencies:
- Package dependent on ``Particle`` version 0.14.
* Miscellaneous:
- Pre-commit hooks added - Black formatting, check-manifest, etc.

* Parsing of decay files (aka .dec files):
- ``DecFileParser`` class enhanced to understand EvtGen's CopyDecay statement in decay files.
* Tests:
- Added tests for Python 3.8 and 3.9 on Windows.
* Miscellaneous:
- Conda badge added to the README, since package now available in Conda.