Why is NumberFieldFractionalIdeal a subclass of Ideal_generic?

A fractional ideal is not an ideal.

This makes about as much as sense as having Rational be a subclass of Integer.

This has been discussed before:

http://groups.google.com/group/sage-devel/browse_thread/thread/0b01d58d8c3565c2/c081ba96b5fed6eb?#c081ba96b5fed6eb

And it came up again recently in #1367.

There seem to be serious design issues with the whole algebraic number theory setup in Sage which make it very frustrating to do any serious work on things like #1367.

CC: @aghitza @pjbruin

Component: number fields

Issue created by migration from https://trac.sagemath.org/ticket/3680