Needs a method to get generated_quantities from the result of an optimization

[dlakelan]

opt = optimize(mymodel,MAP())

genq = generated_quantities(mymodel,opt)

[penelopeysm]

Hmm, this got re-reported as #2607. I'm going to close this one because the other one is more recent, higher up in the issue list, and so we're more likely to pay attention to it 😄