Add a DiskArrays extension

[rafaqz]

As wrapping some kind of DiskArray is a major use case of DimensionalData we may as well move the diskarrays forwarding from Rasters/YAXArrays/PyramidScheme to an extension here, and then they will work in any other wrapper too.

This will mean we can do proper testing of broadcasting and iteration over wrapped DiskArrays here rather than in the extending packages.

For motivation see: JuliaIO/DiskArrays.jl#185

@felixcremer @meggart does that make sense to you? Things are how they are just because extensions didn't exist before, if they did it probably would have been put here from the start.

[felixcremer]

Yes, I think it would make sense to streamline this and make the interaction clearer between Dimensions and DiskArrays.