Adds _test_zonebuilder (we hadn't had any tests for zone_builder before)
Adds pop_Nauru.tif to tests/data
Adds nauru_pop_test to conftest
Fixes _export_hexpopulation as it was breaking previously (it wasn't possible to know that before as we had no tests)
Fixes _hexpop.sql to save MultiPolygons instead of Polygons (after overlaying model area and hex bins, several hexs are cropped and turn to multipolygon)
Fixes _createclusters to be compliant with pygeos removal (now geopandas is working only with rtree). I have added a raise ValueError to avoid situations which a hex bin's population is greater than _max_zonepop. I have also changed the while loop as it wasn't breaking when there were no islands in _removeislands
raise ValueErrorto avoid situations which a hex bin's population is greater than _max_zonepop. I have also changed the while loop as it wasn't breaking when there were no islands in _removeislands