ImageDiff and Structrual Similarity

[aeslaughter]

The ImageDiff test object uses skimage python package to compute the "Structural Similarity" (SSIM). This is a large dependency for a rather simple calculation.

https://en.wikipedia.org/wiki/Structural_similarity

Design

I would like to add a ssim function to mooseutils, so that we can do the following.

import mooseutils
index = mooseutils.ssim('file0.png', 'file1.png')

• The files provided should be read with matplotlib.imread and the function should also
  support a version that accepts the loaded data rather than the filename.
• The ImageDiffer.py:127 can then be replaced with the new function.
• The implementation should include some unittesting.

Impact

This should be drop-in replacement for our current function so no API will change, but it will allow us to remove a large python dependency.

@LSWells

[aeslaughter]

Here is the implementation in skimage, which includes a bunch of stuff we don't need, at least at this point. The only option we used is "multichannel=True"

https://github.com/scikit-image/scikit-image/blob/7875d332f63e1e69b45d8efc2ade97f7a9d14767/skimage/metrics/_structural_similarity.py