Working with target of sky

When the target is not a NAIF body, but instead "sky" what is the plan? The current ISIS labels look similar, but a few things are obviously different (e.g. no POLE_RA, no valid body code for sky, ....)

The issue names no files, entry points, or tests. Start by reviewing how current ISIS labels represent NAIF-body targets and compare the listed sky differences, including POLE_RA and the body code. Done would require an agreed plan for sky-target handling before implementation can be scoped.

Written by the indexing model from the issue text.