new GP function bnrclassfield

[Aurel Page <aurel.page@normalesup.org>]

Dear PARI developers,

I have added a new GP function bnrclassfield, which computes the class 
field attached to an arbitrary subgroup of a ray class group. Internally 
it is a wrapper for rnfkummer.

? bnrclassfield(bnfinit(y^2-y+50)) \\Hilbert class field
% = [x^9 - 24*x^7 + (2*y - 1)*x^6 + 495*x^5 + (-12*y + 6)*x^4 - 30*x^3 + 
(18*y - 9)*x^2 + 18*x + (-2*y + 1)]

By default the class field is expressed as a compositum of cyclic 
extensions of prime power degree:
? bnrclassfield(bnfinit(y^2-y+58))
% = [x^2 + 7, x^2 + (y - 3), x^3 - 3*x + 9]
? bnrclassfield(bnfinit(y^2-y+58),,1) \\as a single polynomial
% = x^12 + 4*x^11 + Mod(7*y - 5, y^2 - y + 58)*x^10 + ... + Mod(3846*y - 
18721, y^2 - y + 58)

With modulus and subgroup:
? bnf = bnfinit(y^3-y^2+1);
? bnr = bnrinit(bnf, [6545,1333,2018;0,1,0;0,0,1]);
? bnrclassfield(bnr, [2,0;0,2])
% = [x^2 + (-5*y^2 + 5*y - 3), x^2 + (-y^2 + 2*y - 5)]

Please test!

Cheers,
Aurel