Load particle definitions directly from the new PDG API

[redeboer]

The PDG recently announced they now offer an API. Their Python API is still under development, but it would be a good idea to get involved in its development, especially given that the particle package presumably can provide good feedback on the needs of the Python HEP community (e.g. can decaylanguage somehow encode the branching fractions that the PDG provides?). In addition, the particle package will probably be a downstream package of the PDG Python package, so we need to provide feedback on that point as well.

[redeboer]

Tagging @juergberinger, who is the main author of github.com/particledatagroup/api.