Separate fitting and forecasting seeds

[swo]

          I think this is worth a separate discussion-- I think we might want separate fitting and prediction keys

Like, I think it's fair for us to fit the model a small number of times (1? 3?) but then we'll want to draw a number of trajectories (100? 1000?).

I want the same fitted model to produce the same trajectory when given the same input key.

There are some nice ways to generate child keys from a parent key.

Originally posted by @swo in #102 (comment)

[swo]

cc @afmagee42 if you have any deep thoughts here