Make optional dependencies?

Documentation lists LHAPDF, nudisx and Geant4 as optional dependencies. I don't know if we can do without Geant, but the first two are needed (if I understand correctly) only for the case when we need to generate neutrino interactions. But they require non-trivial installation, which makes the usage more complicated. And currently they are not optional, but are required by nupropagator package.

Suggestion:

• List nupropagator in the [project.optional-dependencies] section in pyproject.toml, and make sure the code works without it, in case I want to simulate muons.