_buffer_frame for Cartesian index

[CasBex]

It is impossible to apply reduce and related functions over one dimension of multi-dimensional CircularArrayBuffers. Running the following code gives an error:

using CircularArrayBuffers

a = CircularArrayBuffer{Integer}(2, 3)
push!(a, [1, 2])
push!(a, [3, 4])

sum(a, dims=2)
reduce(+, a, dims=2) # both are equivalent

This merge request fixes that behavior.

[findmyway]

Nice, would you mind adding a test case in the meanwhile?

[CasBex]

I added a 2D and 3D test and noticed that it was still not quite working ok, hence 8aab70b. I think it should work now

[findmyway]

Thanks!