Zarr variables don't really implement the DiskArrays interface

[rafaqz]

Broadcasts etc. will fail on zarr variables because they are not <: AbstracDiskArray and don't apply the macros. This is unfortunate, it means users get none of the benefits of DiskArrays that Zarr.jl implements underneath, and requires we hack around it in Rasters.jl

[felixcremer]

This should be closed by #13.