Update run_graphflood documentation

Just tried to run [run_graphflood](https://topotoolbox.github.io/pytopotoolbox/_autosummary/topotoolbox.run_graphflood.html) with v0.07. The function now returns a dictionary and, for example, water depth need to be accessed with `OUT.get('hw')`. I hope I got this right. This will require updates to the documentation.