Labels for show_patches

[mhesselbarth]

show_patches has the option to label all patches using the cell id, however, the id is written in each cell. I think it would be great to either use the centroid, or sample a random cell to display the id only once. Currently its hard to read

[mhesselbarth]

Decided to use a random cell because the centroid can be outside the patch. Improvement would be to use cell which is the nearest to the centroid