Function get_flow_for_year 

fn get_flow_for_year(
    process: &Process,
    target: (RegionID, u32),
) -> Option<Rc<IndexMap<CommodityID, ProcessFlow>>>

Helper function to return a possible flow operating in the requested year

We are only interested in the flow directions, which are constant across years. This function checks whether the process can be operating in the target region and year and, if so, returns its flows. It considers both the commission year and the process lifetime, since a process may operate for years after its commission window. If the process cannot be operating in the target region/year, None is returned.