Model a larger zoo of particles and expand decay methods

[Frencil]

Presently the beta decay method in the nuclide class does an oversimplified form of beta-minus decay, generating only a single electron in addition to the neutron. As decay mode data is folded in (see issue #13) the data will be specific about beta-plus and beta-minus. Full modeling of these decay modes involves particle classes that include two types of neutrinos and the positron.

These particles should be easy to model in the Particle class given how it's been re-written with the introduction of the Nucleus class and the decay method expansion should also be straightforward.