Drop degenerate axes

[confluence]

This fixes a lot of #26, and should be a lot simpler to implement (it probably only requires a change to the metadata and dimensions; no reordering of data needed).

[confluence]

@veggiesaurus has pointed out that dropping the degenerate Stokes axis is a bad plan, because in CARTA 2.0 we want to allow users to read multiple files into one hypercube, and that requires the Stokes information from the header. We should instead just rewrite the header to reorder the axes.