FEAT: publish package on PyPI

[redeboer]

New note to the main webpage:

Each of the pages contain code examples for how to reproduce the results with the Python package hosted at github.com/ComPWA/polarimetry. However, to quickly get import the model in another package, it is possible to install the package from PyPI:

pip install lc2pkpi-polarimetry

Each of the models can then simply be imported as

import polarimetry

model = polarimetry.published_model()