Use DiskArrays.pad in extend when the parent isdisk

[rafaqz]

Waiting on JuliaIO/DiskArrays.jl#219

[rafaqz]

Or possibly add a DiskArrays.pad method as instead? extend doesn't specify a padding value and assumes a writable array is returned.

pad would assume the exact semantics of DiskArrays.pad, and it would work without supplying new dimensions as long as the extended dims are Regular

[asinghvi17]

Extending pad sounds better for the specific use case you want. But we'll have to be very clear when documenting this and PaddedDiskArray should have a nice error when you try to write to it.

[rafaqz]

Yeah, it already has an error message in writeblock!