scale of map to return, one of 110, 50, 10
or 'small', 'medium', 'large'
type
type of natural earth file to download one of 'countries',
'map_units', 'map_subunits', 'sovereignty', 'states' OR the portion of any
natural earth vector url after the scale and before the . e.g. for
'ne_50m_urban_areas.zip' this would be 'urban_areas' OR the raster
filename e.g. for 'MSR_50M.zip' this would be 'MSR_50M'
category
one of natural earth categories : 'cultural', 'physical',
'raster'
full_url
whether to return just the filename [default] or the full URL
needed for download