fill_value should provide default value

[tbenst]

From Zarr spec,

fill_value
A scalar value providing the default value to use for uninitialized portions of the array, or null if no fill_value is to be used.

I think that fill_value = null would correspond to missing in Julia.

Python Zarr also has the following example:

>>> import zarr
>>> z = zarr.full((10000, 10000), chunks=(1000, 1000), fill_value=42)
>>> z
<zarr.core.Array (10000, 10000) float64>
>>> z[:2, :2]
array([[42., 42.],
       [42., 42.]])

Modifying slightly, we can save to disk with z1 = zarr.open_array('/tmp/example.zarr', mode='w', shape=(10,10),fill_value=42). When we open with Zarr.jl,

julia> z1 = zopen("/tmp/example.zarr")
ZArray{Union{Missing, Float64}} of size 10 x 10

julia> z1[1:2,1:2]
2×2 reshape(::Matrix{Union{Missing, Float64}}, 2, 2) with eltype Union{Missing, Float64}:
 missing  missing
 missing  missing

Per spec & python compatibility, this should be [42 42; 42 42]. I'm not sure how hard this would be to change, but I'm happy to look into it if a PR would be welcome? I think this would also resolve #39

[meggart]

Yeah that was a misinterpretation of the specs from my side, where I assumed that FillValues should always represent missing values. I think the fix will be simple, but I would really like to keep an option for the old behavior as a kwarg through something like fillvalueasmissing = false. because using FillValue to encode missing values is a very common use case. Of course this will be breaking, so would be good to fix in conjunction with #78 in case we make a breaking change there as well and then bump the minor version of the package.

I will prepare a fix soon.