cubicweb-postgis #5629998 Add checking and converting functions for Geometry type [open]

Since this cube introduce a new type Geometry, BASE_CHECKERS and BASE_CONVERTERS should be extended to provide a way:

  • to check if a given value is a valid WKT and a valid geometry,
  • to convert a given value from other serialization formats (eg. GeoJSON, GML) into a valid WKT.
done in<not specified>
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