DanRyanIrish

@Cadair this PR is ready for review. To pre-empt possible concerns regarding DKIST tools, the introduction of this new class does not require that metadata be sliceable. Instead it allows...

@Cadair, this is ready to merge as far as I can see. Do you still want more time to look at it given your DKIST concerns? As I above, I...

@Cadair: I've implemented the change to slicing meta that we discussed, https://github.com/sunpy/ndcube/pull/455/commits/cf923f9ce9d45ea9921a544b33b05262ab56a313, i.e. `NDCube` only slices the meta if it has the magic attribute `__ndcube_can_slice__`. Any more concerns before merging...

@Cadair, went looking for this functionality for another package looking to use it and realised we never actually merged it and the stale bot took it. Any objections to merging...

Anyone know what's causing [this warning](https://github.com/sunpy/ndcube/actions/runs/8986257004/job/24682053039?pr=455#step:10:461) in the doc build? The traceback is not helpful. @nabobalis?

This PR should probably also make `NDCube.rebin` check if the `.meta` is an instance of this metadata class and if so, just drop the axis-awareness, without removing the metadata itself.

> This PR should probably also make `NDCube.rebin` check if the `.meta` is an instance of this metadata class and if so, just drop the axis-awareness, without removing the metadata...

Documentation of what axis-aware metadata this can handle needs updating.

@nabobalis The formalisation of "axis-aligned" metadata introduced in this PR, may remove much of the need for `sunraster`'s data classes. `instrument_axes` could simply become a piece of axis-aligned metadata on...

> After we merge this in and we get a release out, should we meet to discuss what to do with sunraster? I'd be happy to have that discussion. We...