Load gpkg (geopackage) from .carto downloaded file

[jaakla]

Probably special service is needed to configure layers, and give SQL-access to geopackage tables from downloaded .carto file. One .carto file can include several datasets (each has own .gpkg file, each with single table), and has one MapConfig.json file with CartoCSS.

Geopackage binary encoding spec: See chapter 2.1.3. Geometry Encoding in OGC Geopackage Encoding Standard document - http://www.opengeospatial.org/standards/geopackage . It is WKB with added own specific header.