Fix aggregation border issues

[rbroth]

Several countries from the GADM dataset have border issues from the union-ing process (slivers). We need to either fix the script used for union-ing the subregions into the whole-country polygon, or else individually download and replace the top-level admin regions for those countries. Countries we know have problems are Zambia, Nigeria and Congo, but others may also have artefacts.

[rbroth]

Note: may not be a problem with GADM, because the country borders we looked at in the meeting are not derived from GADM data. The polygons in the country table are inserted directly by a script edited by @bgsandan (I don't know where he got the data from)

[rbroth]

Looks like the data in the country table are derived from GADM, but were simplified in QGIS before being union-ed, henve the slivers. Re-creating them via a different script should fix them problem

[spenny-liam]

Related to #183