Make it possible to create a skeleton without allocating memory.

[Qfl3x]

The current guide on creating a skeleton:

using YAXArrays, Zarr
a = YAXArray(zeros(Union{Missing, Int32},10,20))
f = tempname();
r = savecube(a,f,driver=:zarr,skeleton=true);
all(ismissing,r[:,:])

allocates memory during the zeros call, which is counter-intuitive to the idea of a skeleton, as well as very memory intensive.

[lazarusA]

please consider doing a PR using FillArrays, see here: #248 (duplicated of this)

[lazarusA]

close by #344