Function: nfisideal
Section: number_fields
C-Name: isideal
Prototype: lGG
Help: nfisideal(nf,x): true(1) if x is an ideal in the number field nf,
 false(0) if not.
Doc: returns 1 if $x$ is an ideal in the number field $\var{nf}$, 0 otherwise.
