Decaylanguage

* 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.

* Dependencies and Python version support:
- Package dependent on ``Particle`` version 0.13.
- Support for Python 3.9 added.

* Dependencies:
- Package dependent on ``Particle`` version 0.12.

* Dependencies:
- Package dependent on ``Particle`` version 0.11.
- Dependencies on `lark-parser` and others upgraded.

* Dependencies:
- Package dependency on ``pydot`` made a requirement.

* Parsing of decay files (aka .dec files):
- Simplifications in various methods of class ``DecFileParser``.
- A couple more tests added.
* Minor fixes.