Function start 

pub fn start(
    chart: &CompiledChart,
    input: StartInput,
    dm: &mut dyn Datamodel,
) -> Result<Outcome, StartError>

Start a session: initialize the datamodel (all data under early binding, top-level data under late binding), run the top-level script, enter the initial configuration and run macrosteps until stable. dm must be a fresh instance of the datamodel the chart names.