code: function(key, point, options) {
          options = options || {};
          var field = {};
          var geo = {
            lat: point.latitude,
            lon: point.longitude,
          };
          var m = {
            order: options.order || 'asc',
            mode: options.mode || 'avg',
            unit: options.unit || 'km',
          };
          m[key] = geo;
          field['_geo_distance'] = m;
  
          this._sortFields.push(field);
          return this;
        }
docs: Add a proximity based constraint for finding objects with key point
  values near the point given.
  @param {String} key The key that the AV.GeoPoint is stored in.
  @param {AV.GeoPoint} point The reference AV.GeoPoint that is used.
  @param {Object} options The other options such as mode,order, unit etc.
  @return {AV.SearchSortBuilder} Returns the builder, so you can chain this call.