Update desi_bayez.py

[NobleKennamer]

@dkirkby @fjaviersanchez

The pull request for updating bayez to the new specsim api is almost completed I just need to finish the update on desi_bayez. I think the main thing that needs to be changed is that the downsample method should be removed from that file entirely and probably placed in another module all together. Should we create something like a helper module to migrate functions like migrate to? What do you think the best approach is for changing this file.

I'm also not sure how these changes should be tested.

[dkirkby]

I suggest moving the downsample function to the existing analysis module.

It should be straightforward to unit test downsample, especially after cleaning up its API. It would also be useful to have some end-to-end tests that integrate the lower-level components.