Fix confusing semantics of model.solutions.load_from

[ghackebeil]

See #365 and #370

[ghackebeil]

Options to consider:

• Tag the results object after the solution was taken out of it by model.solutions.load_from, so that if a user attempts to pass it to model.solutions.load_from again, we can provide a helpful exception.
• Change model.solutions.load_from to leave the solution in the results object after loading it.