IS_CHANGED should be defined as a class method, not an instance method

[curiousjp]

When attempting to use the SystemNotification or PlaySound node, users will see a warning in the log:

WARNING: SystemNotification.IS_CHANGED() missing 1 required positional argument: 'self'

Per custom_nodes/example_node.py.example, IS_CHANGED should be defined as a class method:

    Class methods
    -------------
...
    IS_CHANGED:
        optional method to control when the node is re executed.
...
    #@classmethod
    #def IS_CHANGED(s, image, string_field, int_field, float_field, print_to_screen):
    #    return ""

Fixing this by changing the signature in e.g. system_notification.py or play_sound.py to

@classmethod
def IS_CHANGED(cls, **kwargs):
    return float("NaN")

will fix this error, and also restore the functionality of the sound / notification system. (Just make sure you don't have notifications snoozed...)

I hope you will excuse me not filing a PR on this, but it is such a small change it did not seem worthwhile.

#439 - I believe this will also resolve your issue.