Holes in the inclusion zone are defined by exclusion zones, not by holes
in the inclusion zone. Add a cached property for the inclusion zone that
is not also sea or exclusion zone and use that boundary instead.
Converts the landmap to use MultiPolygon instead of a collection of
polygons, since Shapely has explicit support for this.
Because we've done that, we can use a single projection from a line
instead of brute forcing the extent of the front line.
This makes turn processing ~66% faster (3 seconds to 1.8).
There are probably other places this should be used.
