Add support for reprojecting data cubes with celestial slices using the exact mode

[astrofrog]

Currently we hard-code the exact reprojection to only work with 2d images:

https://github.com/astropy/reproject/blob/master/reproject/spherical_intersect/high_level.py#L63

We should extend this to any data cube that has celestial slices, even though it might be difficult to properly optimize.