dust_system_create - support data.frame argument to pars

[pearsonca]

For doing a parameter scan of some kind, it would be natural to pass a data.frame of parameters to dust_system_create. Obviously possible to convert that into a list of lists (though the transpose is a bit annoying), but would be more user-friendly.

[pearsonca]

FYI, this is what I'm using to get to the required structure:

pars <- do.call(mapply, c(list(FUN=function(...) list(...)), as.list(THE_DATA_FRAME), list(SIMPLIFY = FALSE)))