Decaylanguage

* Parsing of decay files (aka .dec files):
- Fix to parsing of a couple of decay models (subtlety of Lark parsing priorities).
* Miscellaneous:
- CI updates.

* Universal representation of decay chains:
- ``DecFileParser.print_decay_modes()`` enhanced.
- Fix to visualisation of ``DecayChainViewer`` instances in notebooks.
* Miscellaneous:
- CI improvements and updates.

Notebook clean-ups and improvement to ``environment.yml``.

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- Got rid of dependency on ``pydot``, as the package is no longer maintained.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.