[danohu]

For zarr files, set up a .complete marker file to check when data is fully written.

This replaces relying on zarr.json -- that doesn't work because zarr.json is the first to be written. This might happen from a failed or aborted calculation, or because multiple processes are trying to write the same file (e.g. running 2 countries which share a tile)

A partially-written zarr might be not cause any warning signs later, since it might be techncially valid in spite of missing data.

Now an empty .complete marker is written beside the zarr after its last chunk, and the entry counts as cached only once the marker exists. A zarr found without its marker is a write that was killed part-way or is still running: serialize raises FileExistsError and leaves it for someone to resolve by hand, usually by deleting the zarr, rather than writing over it and risking a race with a live writer.

Entries written before this change have no marker, so each raises once until deleted.

Secondary Change -- allow unit tests to run without a .env

Questions for reviewers

What to do when we hit an incomplete zarr? options:

• [what this PR does] raise an exception: conservative, potentially annoying
• delete the partial data: works for detritus from older failed/aborted runs, doesn't deal well with race conditions

This will break existing storage, requiring either a bump of the appropriate versions or to delete the zarr folders. Maybe merge when we're bumping the version anyway?