define "bankfull" elevation in setup_hydrodem half way the river

[DirkEilander]

To better match how the local intertial routing is conceptualized in wflow, the bankfull elevation should be defined at the cell nodes (a.k.a. the river center). The current setup_hydrodem method uses the average cell elevation ("wflow_dem") or subgrid outlet pixel elevation ("dem_subgrid"). The first includes the elevation around the river while the second only looks at the most downstream point. Especially in steep areas there can be a mismatch between both.