normalize method
Normalizes elements to 0, 1 based on min/max scaling.
Returns an empty list if empty or if min == max.
Implementation
List<double> normalize() {
final mn = minOrNull;
final mx = maxOrNull;
if (mn == null || mx == null || mx == mn) return [];
return map((e) => (e - mn) / (mx - mn)).toList();
}