Helper function for finding the center of a polygon?

[C-Loftus]

Thank you very much for your work on this library.

I am trying to get the center of many different geometry types. I wanted to check if there is a function for this already implemented. And if not, if it would be reasonable enhancement. Depending on timeline I could probably look at contributing this upstream. I would probably start with prioritizing geom.Polygon since that is most important to my use case

This is the sort of thing I am looking to accomplish

	geometry, err := wktEncoder.DecodeString(wkt)
	if err != nil {
		return "", err
	}
	switch geometry.(type) {
	case geom.Point:
		point := geometry.(*geom.Point)
		return foo(point.X(), point.Y())
	case geom.Polygon:
		polygon := geometry.(*geom.Polygon)
                // ??
		center := 
        return foo(center.X(), center.Y())
       // so on and so forth for others
	}