Fix 2D index for data

[Djneu]

This is a very small change that updates the data output indexes for compositions in 2D.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.125 ± 0.015 (s=409)	1.125 ± 0.010 (s=393)	-0.3% .. +0.2%
Slab interpolation curved simple none	1.120 ± 0.011 (s=409)	1.119 ± 0.010 (s=397)	-0.3% .. +0.2%
Spherical slab interpolation simple none	1.097 ± 0.009 (s=413)	1.097 ± 0.009 (s=410)	-0.2% .. +0.2%
Slab interpolation simple curved CMS	1.163 ± 0.010 (s=390)	1.163 ± 0.010 (s=386)	-0.2% .. +0.2%
Spherical slab interpolation simple CMS	1.441 ± 0.011 (s=318)	1.443 ± 0.012 (s=308)	-0.1% .. +0.4%
Spherical fault interpolation simple none	1.101 ± 0.009 (s=411)	1.104 ± 0.009 (s=408)	+0.0% .. +0.4%
Cartesian min max surface	2.540 ± 0.041 (s=173)	2.540 ± 0.017 (s=184)	-0.4% .. +0.4%
Spherical min max surface	7.159 ± 0.078 (s=61)	7.172 ± 0.066 (s=67)	-0.4% .. +0.8%

[MFraters]

Thanks a lot for finding this bug! Could you update the test results?

[MFraters]

Thanks! Looks good.