Fixes #87 .Bug in polygon2string conversion

some polygons are made of interior and exterior rings.

methods/Method_D.cpp

@@ -491,6 +491,19 @@ std::string polygon_to_string(const polygon_2d & polygon)
         polygon_str.append(std::to_string(y));
         polygon_str.append("), ");
     }
+    for(auto ring: boost::geometry::interior_rings(polygon) )
+    {
+         for(auto point: ring )
+         {
+              double x = boost::geometry::get<0>(point);
+              double y = boost::geometry::get<1>(point);
+              polygon_str.append("(");
+              polygon_str.append(std::to_string(x));
+              polygon_str.append(", ");
+              polygon_str.append(std::to_string(y));
+              polygon_str.append("), ");
+         }
+    }
     polygon_str.pop_back(); polygon_str.pop_back();  //remove last komma
     polygon_str.append("))");
     return polygon_str;