Print Information About Queued IOTask

[ax3l]

@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 flushed next.

We saw this, because some implementations can accidentally add new operations to the queue.
Our example was In Python: openPMD/openPMD-viewer#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? :)