use `sf` instead of `sp` package in points in polygon tutorial

[hansvancalster]

See #197 (comment)

[hansvancalster]

This issue can be closed. The tutorial already shows how to do this with sf and sp - with the recommendation to use sf.

[florisvdh]

OK. Currently it is expected that sp will be 'retired' around 2024 (I understand that it would still exist but its functions would reroute to sf, in order to keep old code working). At that time we could do another update of all tutorials by dropping remaining sp code.