normal method
Generates a random value from a normal (Gaussian) distribution.
Implementation
double normal(double mean, double stdDev) {
// Box-Muller transform
final u1 = _random.nextDouble();
final u2 = _random.nextDouble();
final z = math.sqrt(-2 * math.log(u1)) * math.cos(2 * math.pi * u2);
return mean + z * stdDev;
}