Add `Nucleus` class which includes GS BE, discrete levels and NLD model from RIPL

[beykyle]

Also include classes for parameteric NLD models (e.g. BFG, KCK systematics)

[beykyle]

Also add a Reaction class which includes projectile and target Nuclei (assumed to be in their GS) and goes to a set of products and a residual, which may be an excited state (e.g. labeled by Ex, J, Pi).