Print Information About Queued IOTask

@AlexanderSinn had the great idea to allow the user to print simple information about the `IOTask` queue that we store in
https://github.com/openPMD/openPMD-api/blob/0.14.4/include/openPMD/IO/AbstractIOHandler.hpp#L136

This would be super helpful to know what will be `flush`ed next.

We saw this, because some implementations can accidentally add new operations to the queue.
Our example was In Python: https://github.com/openPMD/openPMD-viewer/pull/340 .
We found that `data = np.full_like(record_component, np.nan)` probably does something like `record_component[()]` or and another call that queues an IOTask.

In order to debug such situations easier, we could just add a print method to list the tasks and their name (name of the enum).

What do you think @franzpoeschel? :)