Add USA adm2 county polygons

[nvkelso]

Initially 10m scaleset only, 50m maybe later.

[nvkelso]

(This is to track incoming external contribution.)

[nvkelso]

The external contribution arrived, but needs to be processed.

[nvkelso]

/cc @adiku.

[nvkelso]

Credit back to:

• Mapdiva, makers of Ortelius cartography software for macOS
• https://www.mapdiva.com/ortelius/
• on the dataset download page

Also update we have release language to the Terms page:

Natural Earth is hereby granted a non-exclusive, unlimited license to use the data being provided by Mapdiva for the purpose of creating a world base map. Mapdiva makes no claims as to the completeness, accuracy or content of the data, and makes no representation of any kind, including, but not limited to, any warranty as to the accuracy or fitness of the data for a particular use (nor shall the act of distribution constitute any such warranty). No responsibility is assumed by Mapdiva for any claims arising out of Natural Earth’s use and distribution of the data."

Data files:

• ne_10m_admin2_us_counties.zip

[nvkelso]

Fixed via #538