Replace rbind() with bind_sf()

[Robinlovelace]

rbind() is very slow with sf objects: r-spatial/sf#798

We should avoid it on large objects like here:

npt/code/build.R

Line 334 in 2b9a70f

combined_CN_geojson = do.call(rbind, all_CN_geojson)

and here

npt/code/build.R

Line 405 in 2b9a70f

combined_zones_tiles <- do.call(rbind, zones_tiles)

We can use bind_sf instead:

npt/code/build.R

Line 372 in 2b9a70f

network = sf::read_sf(combined_network_file)