function normalize(e) {
        e = e.toLowerCase();
        if (e === 'st_multipolygon') {
          return 'st_polygon'
        }
        if (e === 'st_multilinestring') {
          return 'st_linestring'
        }
        if (e === 'st_multipoint') {
          return 'st_point'
        }
        return e;
      }
this function can only be called during change event
returns true if the geometry type has changed
for this method multipolygon and polygon are the same geometry type