Single file tiff import leads to storage full

[normanrz]

Converting a single tiff file can lead to filling up the storage of a computer. It occurs, when mem-mapping of the tiff file fails.
In that case a new file is created where the data is being copied. That is suboptimal. Even worse, this is happening in the constructor which is called multiple times. I believe the copying should be avoided and the logic in get_frame_2D changed so that the necessary pages are loaded on-demand.