Provide example data with sites of different spatial shapes : (ir)regular polygons, lines, pixels, points

[Rekyt]

Throughout funbiogeo we make the package transparent to work whatever the spatial geometry of the sites in the site_locations object. This is somewhat related to #97 To make sure that all mapping functions are working with these, we should provide example data with locations coming in all types:

• [ ] spaced regular polygons plots of different sizes but not
• [ ] irregular polygons (sites with different geographic extent like different forests patches)
• [ ] polylines (rivers or road sections)
• [x] pixels (our actual example)
• [ ] points (simple locations without any reference to distance apart)