Add nuclei table

[tamasgal]

The SciKit-HEP guys also offer a nuclei table: https://github.com/scikit-hep/particle/blob/master/src/particle/data/nuclei2020.csv

We should add that.

The Python package particle combines everything into a single dataset, so that you can search via

from particle import Particle
Particle.findall()  # lists >6400 particles, including nuclei

I think we should put those in nuclei() or so, but I am not sure, since that makes things a bit more complicated when searching for a particle. We could however also do something like

isnuclei(p::Particle) = ... # 
nuclei() = filter(isnuclei, particles())

but this will re-execute the filter every time.

[tamasgal]

It would probably be a good idea to extend use_catalog_file() with an append=true/false keyword argument.

Which also brings some bad news: we use a dictionary (PDGID => Particle), so that it will overwrite new entries. Maybe we should rethink that and check if the dictionary is really needed? It might be fun to load datasets from 2008, 2017, 2018 etc. and then do some searches on all of them to compare. This also means that each Particle should probably tell from which catalog it comes from 🙈