check on params frame initialization for `flow`

[papsti]

should this this be = values$flow? does it even matter because these are just placeholders or are we accidentally overwriting desired flow rates with the original ones from the model values?

[papsti]

no, because in run-before-simulator.R, a local variable called flow gets created using values (either default or as passed to make_simulator()

[papsti]

nevermind---this might be a problem if we're trying to pass values$flow with make_simulator() (as opposed to in simulate())