Multi-dimensional lookup tables must be passed to ExtraCoords in world order

[Cadair]

This is following on from https://github.com/sunpy/ndcube/pull/327#discussion_r526960030. Due to perfectly sensible design decisions in the LookupTableCoord layer of the new ExtraCoords stack, currently when calling ExtraCoords.add_coordinate is called the lookup_table argument must be in world order if it is multi-dimensional, i.e transposed with respect to the NDCube data array.

We probably want to change this to automatically transpose, but doing that in the add_coordinate method is a little fiddily and needs some thought.

[DanRyanIrish]

@Cadair, is the main challenge here that the ExtraCoords object, just like a WCS, has no knowledge of the data array? And therefore, there's no obvious way for it to interpret array order inputs?

If so, then why not add an add_extra_coord method to NDCube which takes the inputs in array order, uses NDCube's knowledge of both the data and extra_coords to convert them to ExtraCoords world order, and then hand them off to ExtraCoord.add?

At first glance, this might seem like it's cluttering the NDCube API and a self-contained ExtraCoords API is preferable. However, one could argue that cases like this, requiring knowledge of both the data and coordinates, are exactly what NDCube is designed for! And so an add_extra_coord is in fact perfectly within the scope of the NDCube API.

[Cadair]

Note to self: You now have to specify the array axes in world order as well as the array. It's at least consistent but still crazy talk.

[Cadair]

Just a note, as of #414 this is now irrelevant until #416 is fixed.