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)
• pixels (our actual example)
• points (simple locations without any reference to distance apart)