create generic `VectorModel.setup_subbasins` method

[bartvanosnabrugge]

create setup methods for LumpedModel Class. See also #151 .

• [x] setup_region
• [x] setup_subbasins
• [x] setup_downstream_links
• [x] setup_response_unit

setup_region check that setup_region works for lumped/semi-distributed models, and set region to model as response_unit

setup_subbasins wrap functions from here https://deltares.github.io/pyflwdir/latest/basins.html to divide basins into subbasins and add (overwrite) response_unit. Note: this step could also derive stream links.

setup_downstream_links Take the region and subbasins and determine the downstream subbasin of each basin Note: this could / should be extended to include stream links.

setup_response_unit combine above steps in one initial step.

[DirkEilander]

this links to #149