Working examples of mapping all spatial types

[florianm]

Feature

E.g. test form "Locations" (submissions here) contains points, lines (geotrace), and polygons (geoshapes).

While ruODK's focus is to make data from ODK Central accessible, it would be user-friendly to include working examples for the main tasks on spatial data - making maps, spatial operations.

Add a vignette "spatial" to show common operations on spatial field types:

• Parse the WKT or GeoJSON field into an sf::sfc and link to sf / geojsonsf tutorials for further ideas (and as not to take on too much doc maintenance footprint).
• Map points (as shown in vignette odata-api).
• Map lines and polys (both as WKT and GeoJSON).
• Intersect points with shapes (sf::st_join).

What else could be useful?

[florianm]

WIP for vignette: https://rpubs.com/florian_mayer/ruSpatial