code: func (f *FilterOp) GeoDistance(distance string, fields ...GeoField) *FilterOp {
  	f.GeoDistMap = make(map[string]interface{})
  	f.GeoDistMap["distance"] = distance
  	for _, val := range fields {
  		f.GeoDistMap[val.Field] = val.GeoLocation
  	}
  
  	return f
  }
docs: // GeoDistance will add a GEO DISTANCE op to the filter.
  // distance: distance in ES distance format, i.e. "100km" or "100mi".
  // fields: an array of GeoField origin coordinates. Only one coordinate needs to match.