setup graph from hydrography flow directions 1‍⃣

[xldeltares]

For now the graph can be set from dem directly. However, the flow direction is based on cell to cell which is not so reliable, especially for flat area.
Therefore, an extra test, testing if upscaling dem could help in this situation is needed.

• Test: rest flow direction derivation from dem of different scales. #63 --> new issue created

[xldeltares]

Merged into main branch #39

[xldeltares]

Needs improvement: the function needs to be self-contained. See example in setup_urban_sewer_network_from_osm.