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

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.

Start by reading reproject/spherical_intersect/high_level.py around line 63, where exact reprojection is limited to 2D images. Trace how celestial slices in data cubes are represented and determine the existing reprojection entry points. Done means exact mode supports data cubes with celestial slices, with correct behavior even if optimization is deferred.

Written by the indexing model from the issue text.