Release NDCube 2.4.0

[CyclingNinja]

Describe the feature

New release for NDCube needed for the SunPy map refactor wrapping in the new functionality created as part of said refactor in SunPy.

#874 needs merging before the release may happen
#857 outright bug that needs correcting

Proposed solution

Release 2.4.0

[Cadair]

I also want to get the asdf support in 2.4