seir.stochastic: calculates a stochastic trajectory of the S(E)IR model
In sbfnk/dynmod: Dynamic models

This is being used by runSIR.

1	seir.stochastic(parameters, init, seed = NA, transient = 0, tmax = 2128)

`parameters`	parameters of the S(E)IR mode
`init`	initial state
`seed`	random seed. If set to NA, will use R's current seed
`transient`	transient time. If this is >0, the model will be run deterministically for the time specified here before running the stochastic model. Only the outcome of the stochastic model is recorded
`tmax`	the maximum time to which to run the model (form 0)
`init.params`	whether the initial conditions are passed as parameters