Proposed API for NDCube 2.0

[Stuart Mumford]

Hello Everyone,

For a few years now Dan Ryan and myself have been leading a redesign of
NDCube to bring it up to date with the newest features in Astropy and
to make it capable of supporting the DKIST Python user tools which are
going to be based upon it. We are calling this effort "ndcube 2" as it
is a large departure from the current stable version.

As we were working on this large rewrite we quickly realised we would
have to break the API in a way that would be prohibitively hard to
maintain backwards compatibility. Therefore, we are taking the
opportunity to take a fresh look at the choices that were made in the
first version and see where improvements could be made. In addition to
this there has been a long standing plan to have the core sunpy Map
object inherit from NDCube for some time now, and this API rewrite is a
good time to consider the needs of Map.

To these ends, I have written a pretty short SEP with a draft of the
proposed "core" API for NDCube 2. If people who have used, or are
considering using, NDCube could take a look at this it would be great.
Also, if people who are interested in future changes to Map could take
a look and see what you think of the possibility of a Map with the
methods proposed in this SEP.

You can find the SEP here: https://github.com/sunpy/sunpy-SEP/pull/51

While we are still a while away from having a ready to release version
2 of NDCube, feedback at this point would be really helpful before
significant effort has been invested in implementing some of the things
proposed in the SEP.

Thanks as always,
Stuart