Update the vector data

[perrette]

We currently (v2.8 and previous versions) use the GAUL dataset for the vector, with some input from Natural Earth, as documented in the README. The country polygons are encoded individually (geojson format) and partially overlap in some areas (this is also the case in the original dataset). This may lead to innacuracies in high resolution country masks such as 5" and 30". More generally, the vectors were originally selected for ISIPEDIA and did not undergo a systematic review for adequacy for high-resolution masks (a relatively recent request). Here below a list of vector data providers to review for the next round of update:

• GAUL-ASAP (current)
• Natural Earth
• geoboundaries.com
• World Food Programme (UN agency)
• GADM