average method
Returns the average or null
if there are no elements.
Implementation
double average() {
var count = 0;
num sum = 0;
final i = iterator();
if (!iterator().hasNext()) return double.nan;
while (i.hasNext()) {
final next = i.next();
// nan values are ignored
if (!next.isNaN) {
sum += next;
count++;
}
}
return sum / count;
}