average abstract method
Computes the average of some ManagedObject property.
selector
identifies the property being averaged, e.g.
var query = Query<User>();
var averageAge = await query.reduce.average((user) => user.age);
The property must be an attribute and its type must be an num, i.e. int or double.
Implementation
Future<double?> average(num? Function(T object) selector);