Add a `radius` argument to `find_snap()`

[dfsnow]

This PR adds a radius argument to r5r's find_snap() function. radius controls the maximum search distance when snapping points in a data.frame to the OSM road network.

The default search radius set in R5 is just 1600m, which can be frustratingly small when working with large matrices in a grid (or other shape whose centroids may not align with the road network).

Note that this argument is already present in r5py.