Add a "radius" parameter in oe_get

[agila5]

Idea: add a radius argument to one of the functions used by oe_get that can be used to filter all features intersecting a buffer with a given radius centred in a given point. Something like:

oe_get(c(9, 45), radius = 0.025)

or

oe_get("Leeds, UK", radius = 0.025)

The string is converted to a pair of lat/long coordinates via OSM Nominatim and then the given point is buffered according to the given radius.

[Robinlovelace]

This is a great feature request that I have needed. Great plan, you can get most parts of most cities with a 30 km radius:

Source: https://zonebuilders.github.io/zonebuilder/articles/paper.html