replace center average with geometric center. fixed #284

src/geo/polygon.js

@@ -615,7 +615,8 @@ class Polygon {
      * @returns {Polygon|Point} a new polygon centered on x=0, y=0, z=0
      */
     center(point) {
-        throw "does not work";
+        return this.bounds.center(this.getZ());
+        // throw "does not work";
         let ap = newPoint(0, 0, 0, null),
             np = newPolygon(),
             pa = this.points;