[FEA] Modify `read_polygon_shapefile` to return a `GeoSeries` instead of a tuple.

[thomcom]

Describe the bug read_polygon_shapefile returns the basic elements of a GeoSeries instead of a GeoSeries. To solidify the API, let's have it return the GeoSeries itself.

[harrism]

What is the C++ equivalent of a GeoSeries?

[thomcom]

I'm really referring to the python interface in this issue, which was my oversight. There isn't a parallel for GeoSeries yet at the C++ level.

In the general case the C++ equivalent is: A buffer of xy Points A buffer of xy Points and an offsets buffer of MultiPoint parts A buffer of xy coordinates, an offsets buffer of LineString start positions and an offsets buffer of MultiLineString parts A buffer of xy coordinates, an offsets buffer of Polygon rings, an offsets of Polygon start positions, and a buffer of MultiPolygon parts.

read_polygon_offsets returns the first three objects in the Polygon set.