Enrich implementation methods

[FusionSandwich]

Currently, I don't know how others want the function enrich() to be implemented. I think that the extra testing requirements to have enrich() able to be called with mix_by_volume(), make_mat_from_atom(), and make_mat() is worth it. Since users will still need to specify the material, atom, and isotopes to be enriched. That should provide adequate context for users to not be confused with their enrichments, while still providing a large amount of flexibility.