Approach to Null value handling

[TomNicholas]

I'm not sure how to think about handling null values in the context of ManifestArray. Partly because I'm not very familiar with how zarr / zarr-python handles them.

Relevant for #16, #22 and #29.

[rabernat]

Zarr has the concept of a fill_value. This value is used in the case where a chunk is not found in the store to represent missing data.

Zarr has no concept of an arbitrary null value mask (like Xarray / CF conventions). When you use Zarr with Xarray, Xarray uses its own decoding / encoding mechanism to translate fill_value to NaN.

[TomNicholas]

Thanks.

This value is used in the case where a chunk is not found in the store to represent missing data.

This answers #16, and means that the code I wrote to check that every chunk in the grid exists in the manifest might have been too conservative...

https://github.com/TomNicholas/VirtualiZarr/blob/28e02ce704b01f93edb8d8fb854f74dfbf9aaacd/virtualizarr/manifests/manifest.py#L169