Square roots and nth roots in number fields

[fredrik-johansson]

There is a WIP implementation of square roots in flintlib/antic#51 which apparently needs some more work.

[fredrik-johansson]

The best thing to do might just be to implement polynomial factorization over $\mathbb{Q}(a)$ with the standard algorithm. Then one can just compute nth roots by factoring $x^n-c$.