code: public PointExpression<Point> centroid() {
          if (centroid == null) {
              centroid = GeometryExpressions.pointOperation(SpatialOps.CENTROID, mixin);
          }
          return centroid;
      }
docs: The mathematical centroid for this MultiSurface. The result is not guaranteed to be on
  this MultiSurface.
  
  @return centroid