Add Custom Objective Function Terms into epl.Accounts

[ADGEfficiency]

@johnfeldhausen - I've started work on bringing the custom objective function terms into epl.Accounts here #65.

The initial idea is something like:

simulation = site.optimize(verbose=4, objective={"terms": terms})
accounts = epl.get_accounts(simulation.results, terms=terms)
print(accounts.custom.cost)

Will see how it goes!

[ADGEfficiency]

Implemented the feature in #65

Still needs documenting - the epl.Accounts aren't documented at all, so will set that up as well.