Missing non-celestial reprojection routines

The [documentation](https://reproject.readthedocs.io/en/stable/noncelestial.html) claims "Non-celestial images/data are not currently supported but will be added in future releases." but there's no open issue about this that I can find.

We need to support reprojection between different position-velocity slices.

Start with the noncelestial.html documentation linked in the issue and inspect the repository's current support boundaries. Define the scope for reprojection between different position-velocity slices and identify tests that demonstrate support; the work is done when that use case is supported and documented.

Written by the indexing model from the issue text.