Implement abs and sqrt for UniversalCyclotomicField elements

[slelievre]

Inspired by this report by Ask Sage user Assombrance:

• ​Ask Sage question 44623: Norm in UniversalCyclotomicField

For abs, we need to implement __abs__ for elements of the universal cyclotomic field; it could be computed as the square root of z * z.conjugate() or of z.real()**2 + z.imag()**2, or by using to_cyclotomic_field, or by changing ring to QQbar.

[chapoton]

New commits:

​b7f0b12	new method "is_integral" for elements of universal cyclotomic field
​db44a9e	trac 26872 adding abs and sqrt to universal cyclotomic field

[chapoton]

green bot, so this seems to be ready for review

[tscrim]

This will work well as a first implementation. If we need to optimize either of these operations later on, then we can do it then.

[slelievre]

Oops. The method should have been called __abs__ rather than abs.

Follow-up at #27201.