Add convenience coordinate methods to NDCube

[DanRyanIrish]

PR Description

This PR adds 4 methods for getting the certain common coordinate types for all array indices of relevant axes:

• NDCube.celestial
• NDCube.time
• NDCube.spectral
• NDCube.stokes

These are convenience methods for NDCube.axis_world_coords. If the WCS does not contain the relevant axis, None is returned.

[Cadair]

I have many thoughts on this.

[DanRyanIrish]

I have many thoughts on this.

I knew you would :) I look forward to them.

[DanRyanIrish]

I have many thoughts on this.

@Cadair Would your thoughts be something more along the lines of #710 ?

[Cadair]

Yes that's a starter.

[DanRyanIrish]

Yes that's a starter.

Well if we can agree the API, at least for coords.celestial, coords.spectral and coords.time, we can push ahead with that and change the underlying implementation at a later time, as well as expand the API as desired.