Consider adding the SSIM statistics to the output of MET

[JohnHalleyGotway]

Describe the Enhancement

MET #3024 adds new columns to gradient line type (GRAD) from Grid-Stat to help quantify "sharpness", especially in AI/ML output, using the methods defined in this paper:
An Investigation of Metrics to Evaluate the Sharpness in AI-Generated Meteorological Imagery

That paper also mentions SSIM statistics:

• Structural Similarity Index Measure (ssim) is a similarity measure between two images based on a weighted combination of three simpler comparisons: luminance, contrast, and structure. If this were well-defined, we could consider adding it to the CNT line type.
  See reference on Wikipedia

Recommend considering adding the SSIM statistic, along with its luminance, contrast, and structure components.

These could be added to the existing continuous statistics (CNT) line type, but more thought should be given to where it belongs.

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