cleanup raster processing notebook 13

[stijnvanhoey]

Some sections are now a bit redundant to show clip and ``rasterize; we should improve the structure to make clear what the repitition is and what is new.

• mask_and_scale has already been tackled before -> shorten in this notebook (just a recap)
• it is a lot of clipping no (both on initial section as on dem example); might be better to stick to 1 of those or just start from the clipped dem directly to have the focus on the rasterize only.

[stijnvanhoey]

@jorisvandenbossche I kept this mostly as such for the moment, but I'm open to adjust it and shorten the section

[stijnvanhoey]

see plovercode/course-python-geospatial#65

it is a lot of clipping no (both on initial section as on dem example); might be better to stick to 1 of those or just start from the clipped dem directly to have the focus on the rasterize only.

Due to the big data size of the DEM (even after clipping it is 30MB) and with massive downsampling we do miss the effect of the burning rivers, I do prefer doing the manual data download and keep the processing steps.
I put them in intermezzo section for data preparation so we can skip the explanation and let just everyone run the section

mask_and_scale has already been tackled before -> shorten in this notebook (just a recap

removed redundant repetition