Store EffHam terms as a dictionary instead of a list?

[matt-chan]

It would be nice to store the effham terms in a dictionary of {label:term} style. Right now it's impossible to retrieve a term by label, without iterating through the whole list and checking, or by inspecting the cache dictionary (which may be stale?).

I don't know if we actually want to expose the terms in the hamiltonian though, so there's that...