improve module 9 network theory

[maze2point0]

Some things that came up at the last training

based on the slides https://docs.google.com/presentation/d/1iz8-dypUhwLPDRj70ju2FMEGF4MFaXsbF_gotBTrz4I/edit?usp=sharing

• [ ] add images from slide 9 as examples of raw road network and the conversion to network (nodes / edges)
• [ ] add more about the why networks. This can be based on the problems on slide 11
• [ ] More on accessibility as a subtopic of network analysis.. maybe as a separate file/chapter.
• [ ] Isochrone based (e.g. 34, 39)
• [ ] potentials with other weights (traffic & ocngestion) slide 40
• [ ] access during floods (39, https://pubmed.ncbi.nlm.nih.gov/36224567/)
• [ ] raster explanations: 36 & examples in 37