add a DiskArrays extension so that Zarr arrays are fast

[bjarthur]

it's orders of magnitude faster. still working on getting your benchmark scripts to work on my machine. let me know what you think and i'll add tests and docs.

[bjarthur]

here's a little ad-hoc benchmark script:

using DiskArrays, ImageMorphology, Zarr
a = rand(Bool, 256,256,256);
z = ZArray(a);
oa = similar(a);
oz = similar(z);
se = strel_diamond((3, 3, 3));
erode!(oa, a, se);
erode!(oz, z, se);

you have to manually run it on master and this PR. on master it will essentially hang forever on the last line. returns essentially immediately with this PR.

[bjarthur]

@rafaqz