Test case: assert statement not detailed enough

[haesleinhuepf]

Hi Riccardo @rmassei,

just some feedback about a test-case in #142 I didn't merge, because the assert seems not detailed enough. The test rescale_image should also test some intensity values in my opinion.

rescale_image.ipynb.txt

See also subsample_image.

Feel free to refine the test-case and send a new pull-request.

Thanks for your time!

Best,
Robert