Added first round of unit tests for qim3d.io.load()

Not sure which other tests to include as long as we only have TIFF files in the img_examples. Should we add TXRM and H5 files to the examples as well so I could test for those filetypes as well?

I have also updated the docstrings in load.py to be in correspondence with the docstring in save.py. If you think it should be different, feel free to change it, but do it in both files.