Move r4ss into nmfs-stock-synthesis organization?

[iantaylor-NOAA]

Moving the r4ss repo (and potentially ss3sim) into the nmfs-stock-synthesis organization (which didn't exist when the r4ss organization on github was created) could consolidate some of the overhead associated with having these related repositories in separate places.

Checklist below copied from nmfs-ost/ss3-workflows#30.

• move over the r4ss/shiny-selex repo
• Learn from this experience to decide the details of how to migrate r4ss
• Clean up other r4ss org repos that aren't used (e.g., r4ss::SS_utils can probably just be deleted; likely true for other repos)
• Move r4ss over to nmfs-stock-synthesis
• Announce move of r4ss in release notes
• Decide on strategy for redirecting folks to the new location of r4ss (e.g., looking at apps that depend on r4ss and letting them know about the change? Potentially forking the new location of r4ss back to the old and adding a message upon trying to download or use r4ss from the old location?