Build out IsotopeLibrary

[jccurtis]

We have discussed adding/extending the features of the isotope lib including:

• [ ] auto caching or including a cached version of the wallet cards in the repo for offline/testing use (is this a licensing issue?)
• [ ] creating a standard for isotope naming
• [ ] creating a unique encoding for isotope+energy entries (maybe hashing or <name>_<int_energy_in_keV>_<int_branching_ratio>)
• [ ] adding dose rate conversion factor(s) @markbandstra 👀
• [ ] file io (yaml, json, h5)
• [ ] ... ?