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