Allow different function field implementations to have different differential classes

[BrentBaccala]

This code adds an Element to FunctionFieldDifferential and makes some simple changes so that element_class() is used to construct differentials.

This allows function field implementations to define custom classes for their associated differentials.

Depends on #26991
Depends on #26993

Component: algebra

Author: Brent Baccala

Branch/Commit: public/27220 @ 64a37f6

Issue created by migration from https://trac.sagemath.org/ticket/27220

[BrentBaccala]

Author: Brent Baccala

[BrentBaccala]

Branch: public/27220

[BrentBaccala]

Commit: 64a37f6

[BrentBaccala]

New commits:

d3b7833	Add global function fields
f59d184	Trac #26991: refactor divisor methods
d5d2ab0	Trac #26991: refactor 'place' method into ideal super class
413aa01	Trac #26991: refactor 'space_of_differentials' method in FunctionField
6642cfd	Trac #26993: allow mult of function field elements and differentials
64a37f6	Trac #27220: allow different implementations to have their own differential classes

[BrentBaccala]

Dependencies: #26991, #26993

[BrentBaccala]

comment:3

This was already done in 3b4fcb8.