handle trajectories on server side

[mdip226]

`if seed is None: if number_of_trajectories > len(existing results):

run more, add to result

else:
   # return cached result`

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Just to make sure I got it, we don't really mind returning more trajectories than asked for, right? I could pick them out and just return what they asked for. I think that is what we want. It's been a while since we talked about this part so I'm jotting this down to make sure I remember to bring it up