Explicitly setting fill-values

[jeromekelleher]

Some weird behaviour in xarray means that we need to think about explicitly setting fill values for all our arrays:

xref: #313 (comment)

Explicitly setting to None seems easy enough, but I just want to double check what the semantics of this is. We don't use empty chunks, and I guess if we did we'd want to use the -1 value instead (but it would likely be quite subtle, with dimension padding on many of our arrays).

What does a fill_value of null really mean in Zarr?

[benjeffery]

What does a fill_value of null really mean in Zarr?

My understanding is that a null fill_value means that the content of missing or uninitialised chunks is undefined and implementation-dependent. So would only be an issue for us if a chunk was missing on disk.