Use git lfs better

[denised]

The file .gitattributes is set up to have some files in git lfs, but not all that could be (it currently misses all the zips). It would be a simple change to have git lfs function for all Excel and zip files, but before doing that I want to understand the implications better. In particular, we need to test the behavior experienced by a user who does not have git lfs installed and clones the repo. Since the files that we use git lfs for are (currently) only used for testing, it would be acceptable if a "casual" user did not get the actual binary files, but not acceptable if they got an error message that tells them their download or clone failed. And it would be unacceptable if an initial (apparently successful) download or clone threw up errors later when the user did push/pull/rebase or PR request, or if they later did install and use lfs.

Documentation about lfs is surprisingly sparse and this is too risky to do without thorough testing.