updates cluster build

[r-akemii]

• Adds test_zone_builder (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_hex_population as it was breaking previously (it wasn't possible to know that before as we had no tests)
• Fixes hex_pop.sql to save MultiPolygons instead of Polygons (after overlaying model area and hex bins, several hexs are cropped and turn to multipolygon)
• Fixes create_clusters 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_zone_pop. I have also changed the while loop as it wasn't breaking when there were no islands in remove_islands