distance method
Returns a distance
Implementation
double distance(GeoPoint other, {double radius = GeoPoint.earthRadius}) {
final sDLat = sin((other.latitude.toRad() - latitude.toRad()) / 2);
final sDLng = sin((other.longitude.toRad() - longitude.toRad()) / 2);
final a = sDLat * sDLat + sDLng * sDLng * cos(latitude.toRad()) * cos(other.latitude.toRad());
final c = 2 * atan2(sqrt(a), sqrt(1 - a));
return radius * c;
}