notify_once: Print a warning message if not printed earlier
In emil: Evaluation of Modeling without Information Leakage

To avoid flooding the user with identical warning messages, this function keeps track of which have already been shown.

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notify_once(id, ..., fun = log_message)

reset_notification(id, if_top_level = TRUE)

`id`	Warning message id. This is used internally to refer to the message.
`...`	Sent to `warning`.
`fun`	Function to display the notification with. Typical choices are `message` or `warning`.
`if_top_level`	If `TRUE` the notifications will only be reset if `reset_notification` was called from a top-level function call. This behaviour prevents the notifications from being reset multiple times during nested calls to functions such as `fit` and `evaluate`.