new function: galoisexport

[Bill Allombert <allomber@math.u-bordeaux.fr>]

Hello PARI-dev,

I have added a new GP function galoisexport() that 
export the underlying permutation group from the 
galoisinit() output to a format suitable for GAP
(default) or Magma (flag=1).

Let's me recall that if G=galoisinit(K) the automorphisms
(G.group) are stored as permutations of the roots G.roots
modulo G.mod. Generators of this group are given by
G.gen.

Here an example:

? G=galoisinit(x^6+108);
? G.gen
%2 = [Vecsmall([2, 3, 1, 5, 6, 4]), Vecsmall([4, 6, 5, 1, 3, 2])]
? s=galoisexport(G)
%3 = "Group((1, 2, 3)(4, 5, 6), (1, 4)(2, 6)(3, 5))"
? extern("echo \"IdGroup("s");\"| gap -q")
%4 = [6, 1]
? galoisexport(G,1)
%5 = "PermutationGroup<6|[2, 3, 1, 5, 6, 4], [4, 6, 5, 1, 3, 2]>"

Cheers,
Bill.