Add new SI prefixes

[namurphy]

Description

For the first time since 1991, new SI prefixes have been created. 🎆 ⚖️ 🎉 🎂 🎈 🌍

• quetta (Q): $10^{30}$
• ronna (R): $10^{27}$
• ronto (r): $10^{-27}$
• quecto (q): $10^{-30}$

This PR adds these prefixes to astropy.units.core, as well as to the corresponding table in the documentation for astropy.units. Here's hoping that there aren't any namespace clashes...

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the Astropy coding guidelines?
• Are tests added/updated as required? If so, do they follow the Astropy testing guidelines?
• Are docs added/updated as required? If so, do they follow the Astropy documentation guidelines?
• Is rebase and/or squash necessary? If so, please provide the author with appropriate instructions. Also see "When to rebase and squash commits".
• Did the CI pass? If no, are the failures related? If you need to run daily and weekly cron jobs as part of the PR, please apply the Extra CI label. Codestyle issues can be fixed by the bot.
• Is a change log needed? If yes, did the change log check pass? If no, add the no-changelog-entry-needed label. If this is a manual backport, use the skip-changelog-checks label unless special changelog handling is necessary.
• Is this a big PR that makes a "What's new?" entry worthwhile and if so, is (1) a "what's new" entry included in this PR and (2) the "whatsnew-needed" label applied?
• Is a milestone set? Milestone must be set but astropy-bot check might be missing; do not let the green checkmark fool you.
• At the time of adding the milestone, if the milestone set requires a backport to release branch(es), apply the appropriate backport-X.Y.x label(s) before merge.

[github-actions]

👋 Thank you for your draft pull request! Do you know that you can use [ci skip] or [skip ci] in your commit messages to skip running continuous integration tests until you are ready?

[mhvk]

Ha, that's news! I think it is all OK -- there don't seem to be any units starting with Q or q, and only the various radii, radian and Rayleigh with R. So, no confusion with, say a duration in units of "Ra".

I think we should backport this, so that all supported versions fully support SI.

[namurphy]

Ha, that's news!

I know, right? Now we can say that Earth's mass is $∼ 6$ ronnagrams and Jupiter's mass is $∼ 2$ quettagrams. The prefixes aren't quite enough to deal with the range of stellar/galactic masses...but I guess it's the thought that counts?

I think it is all OK -- there don't seem to be any units starting with Q or q, and only the various radii, radian and Rayleigh with R. So, no confusion with, say a duration in units of "Ra".

That's right — I did a check to see if there would be any namespace conflicts and I didn't find any:

>>> [unit for unit in dir(astropy.units) if unit[0] in "qQrR"]
['Quantity', 'QuantityInfo', 'QuantityInfoBase', 'R', 'R_earth', 'R_jup', 'R_jupiter', 'R_sun', 'Rayleigh', 'Rearth', 'Rjup', 'Rjupiter', 'Rsun', 'Ry', 'quantity', 'quantity_helper', 'quantity_input', 'rad', 'radian', 'rayleigh', 'rydberg']

I think we should backport this, so that all supported versions fully support SI.

Cool! Is that something the maintainers will take care of, or should I submit separate PRs to the relevant branches?

[mhvk]

Sounds all good! The backport should be automatic -- which we will see whether it works in a second, because I think I should just press that nice "merge" button! Thanks, @namurphy!

[pllim]

Power of 27? Why? 🤯