AutoRIFT should quickly fail if the reference and secondary scenes are the same

[jhkennedy]

AutoRIFT is a pixel-tracking algorithm and so requires at least some difference between the reference and secondary scene.

Unfortunately, autoRIFT will process a pair that has the same scene for reference and secondary all the way through and then finally fail due to NaN issues in the netCDF packaging step at the very end, like in this job:
https://hyp3-api.asf.alaska.edu/jobs/879efe81-adf2-48cb-89db-f4bd88c577e4

We should check up front that the reference and secondary scene provided aren't exactly the same and fail immediately.