code: func (ps PointSet) ToGeoJSON() *geojson.Feature {
  	f := geojson.NewMultiPointFeature()
  	for _, v := range ps {
  		f.Geometry.MultiPoint = append(f.Geometry.MultiPoint, []float64{v[0], v[1]})
  	}
  
  	return f
  }
docs: // ToGeoJSON creates a new geojson feature with a multipoint geometry
  // containing all the points.