Better ordering in all the modes

So I rearraged the different modes. I did not make different classes as
@tacaswell advised, mainly because I don't know exactly how. I
introduced the variable 'mode' that can have several options, selecting
the get_frame routine to use. In addition only the __len__, sizes and
pixelsizes properties have different functionalities per mode.

My nd2 files are often misinterpreted by bioformats; 2 modes cover those
errors. However this should be solved on the bioformats side. I am
trying to make a ticket for this error at the OME fora.