Compare (difference) output prediction rasters

Add a new function to compare output prediction rasters, i.e. create a difference raster to see where models are agreeing (or not).

No file, test, or entry point is named. Inspect the package's existing code for producing output prediction rasters and its example data, then determine how a difference raster should represent agreement and disagreement between model outputs. Done means a new comparison function produces the requested difference raster.

Written by the indexing model from the issue text.