Downloads a PNG from the stamenmap tile server and creates a
raster::rasterBrick
object with layers for red, green, and blue. This
can then passed as the rasterBrick
object to the
staticmap_plotRasterBrick()
function for plotting.
Stamen maps tiles are freely available (April, 2019) and are described at the
following URL:
http://maps.stamen.com/#terrain/12/37.7706/-122.3782
"These tiles are made available as part of the CityTracking project, funded
by the Knight Foundation, in which Stamen is building web services and open
source tools to display public data in easy-to-understand, highly visual
ways."