Function: nfsubfields
Section: number_fields
C-Name: subfields0
Prototype: GDG
Help: nfsubfields(nf,{d=0}): find all subfields of degree d of number field
 nf (all subfields if d is null or omitted). Result is a vector of subfields,
 each being given by [g,h], where g is an absolute equation and h expresses
 one of the roots of g in terms of the root x of the polynomial defining nf

