fix for ssim

[sfarrens]

• Updated SSIM with latest changes to Scikit-Image.
• Updated corresponding test.

[zaccharieramzi]

Minor thing

[chaithyagr]

Also, a quick question, does this answer things discussed in CEA-COSMIC/pysap#111 (comment) ?
I am asking this as there is a probability that if the PR CEA-COSMIC/pysap-mri#30 goes through (which it might soon), there could be some changes to examples.
If there are more changes, I could just get another release of pysap-mri

[sfarrens]

@chaithyagr This fixes the part b) of my comment. I would prefer not to add Scikit-Image as a mandatory requirement, therefore you should probably add a check to the example to see if Scikit-Image is installed before calculating the SSIM.