move finite field methods

[mantepse]

• move is_field from Ring to Rings
• adapt doctest
• assert that a coercion cannot be registered after a conversion has been discovered, disable check for zero ring in is_field
• move Ring.zeta and Ring.zeta_order
• adapt doctests
• move is_perfect, zeta, zeta_order

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 2d80c18; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[fchapoton]

This now looks good. I have committed my own small suggestion. Good to go

[mantepse]

I am currently sick, but it seems to me that we haven't found a good solution to the problem with cellularbasis.one yet.

[mantepse]

OK, so is there anything left to be done here?

[fchapoton]

well, the continuous integration seems to be very broken right now. So maybe one needs to run the full tests ourselves..

[mantepse]

I'm doing that now, on ubuntu.

[mantepse]

On Ubuntu 24.04.2 LTS all tests pass, except for two expected FriCAS failures.

[mantepse]

So, the only question is whether we are doing the right thing here, design-wise. I cannot judge this.

[fchapoton]

This is moving in the right direction, so positive review.