Make AccessCountDiskArray <: AbstractDiskArray + fix zip bug

[charleskawczynski]

I'm hoping that this will alleviate or close #175.

[charleskawczynski]

Oh, wait, @rafaqz, can we just move the TestTypes module outside of source? Why would DiskArrays ship that anyway?

[felixcremer]

We use TestTypes in higher level packages like YAXArrays to ensure that code is IO efficient.

[charleskawczynski]

We use TestTypes in higher level packages like YAXArrays to ensure that code is IO efficient.

Couldn't that just be shipped as a library instead?

[felixcremer]

There might be a way to put this in an extension on Test, but then we would need to get the types out of the extension.

[charleskawczynski]

There might be a way to put this in an extension on Test, but then we would need to get the types out of the extension.

Yeah, an extension would work, or even just make a DiskArraysTestTypes package in this repo, and then just depend on that where needed. I don't think that this is a blocker for this PR, but I think moving it to a separate package is a cleaner solution entirely.

[charleskawczynski]

I think I found a bug!

function Base.zip(A1::AbstractDiskArray, A2::AbstractArray, As::AbstractArray...)
    return DiskZip(A1, A1, As...)
end

should be

function Base.zip(A1::AbstractDiskArray, A2::AbstractArray, As::AbstractArray...)
    return DiskZip(A1, A2, As...)
end

! This is probably pretty confusing since it was correctly implemented in implement_zip! I also removed the DiskZip(As...) method because we already check dimensions in DiskZip(As::AbstractArray...).