Reads the next dynamic item-timestep. First time called it returns the first timestep of the first item. It cycles through each timestep, and each item in the timestep. For a file with 3 items, it returns (itemnumber, timestepIndex) in the following order: (1,0), (2,0), (3,0), (1,1), (2,1), (3,1), (1,2), etc.

If one of the ReadItemTimeStep is called with for example (1,4) the next call to this method will continue from there and return (2,4).

If one of the methods reading/writing static item data is called, the iterator is reset, and the next call to this method again returns the first item-timestep.

This is the most efficient way to iterate through all the items and timesteps in a file, since it iterates exactly as the data is stored on the disk.