Refactor convex hull calculation to treat multipolygon districts individually

[mxndrwgrdnr]

The convex hull code currently merges all points of a multipolygon (i.e. a group of islands or a district that includes an island) and finds the area of the convex hull for all points at once. This greatly exaggerates the area of the convex hull, which should instead be the summation of the areas of the convex hulls of the individual polygons included in the multipolygon.

[r-barnes]

The cpp branch, which is now on master, does this.