Merge topic 'image-ssim'

7bd8216f Adding docs for new image metric
58277762 baseline commit
a544f58d aliasing cmake commit
a8b4066a Tests: setting new threshold for baseline to 0.05
405a52f9 Updating vtkTesting to use SSIM
b6a11edb Defining various env and CMake variables for image testing
8b0f1c8f Adding an SSIM computation filter for images
446f4342 Adding image filters to convert RGB to XYZ and XYZ to Lab

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !10508