Have a function in Provider to reinstantiate the class

[Jiehfeng]

I believe this is would be a really good feature to have. Instead of getting us to make a method to set values to their defaults, why not create a method that can be called from context just like when reading or watching, to reinstantiate the instance of the class used by ChangeNotifierProvider?

The classes we create have their own defaults set when it gets instantiated, so a method from the provider to reinstantiate it to 'reset' everything back would be very helpful and time saving.