Fix slicing and axis_world_coords for extra coords that span several axes

[nabobalis]

PR Description

Slicing (ExtraCoords._getitem_lookup_tables): table axes given as a list were treated as a single axis, so any slice raised TypeError. ASDF stores the axes as a list, so this broke every such table loaded from ASDF. Integer indexing also kept the dropped axis ((0, 1) became (-1, 0)), which gave the wrong length, or NaN, on a cube with more axes than the table.

Transposed output (_generate_world_coords): correlated world coords were transposed with a plain .T. That is only right when the WCS pixel inputs are in ascending cube-axis order. This holds for the cube's WCS, but not for an ExtraCoords WCS, whose inputs follow ExtraCoords.mapping. So axis_world_coords(wcs=cube.extra_coords) transposed a 2-D table on (0, 1), and also WCS-backed extra coords whose mapping reorders correlated axes. The transpose now follows the mapping. For the cube's own WCS it is still a full reversal, so normal WCS output is unchanged; the existing axis_world_coords tests check that.

import numpy as np, astropy.units as u
from astropy.coordinates import SkyCoord
from astropy.wcs import WCS
from ndcube import NDCube

lon = np.arange(12.).reshape(3, 4)
for axes in [(0, 1), [0, 1]]:
    cube = NDCube(np.zeros((3, 4, 5)), wcs=WCS(naxis=3))
    cube.extra_coords.add(("lon", "lat"), axes, SkyCoord(lon * u.deg, lon * 0 * u.deg), mesh=False)
    for item in [np.s_[:], np.s_[1]]:
        try:
            sliced = cube[item]
            out = sliced.axis_world_coords(wcs=sliced.extra_coords)[0].ra.deg
            print(axes, item, out.shape, "ok" if np.array_equal(out, lon[item]) else "WRONG")
        except Exception as e:
            print(axes, item, type(e).__name__, e)

main:

(0, 1) slice(None, None, None) (4, 3) WRONG
(0, 1) 1 (5,) WRONG
[0, 1] slice(None, None, None) TypeError list indices must be integers or slices, not list
[0, 1] 1 TypeError list indices must be integers or slices, not list

this branch:

(0, 1) slice(None, None, None) (3, 4) ok
(0, 1) 1 (4,) ok
[0, 1] slice(None, None, None) (3, 4) ok
[0, 1] 1 (4,) ok

Related to #342

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• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
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• No AI tools were used

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