ellrank and Cassels pairing

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

Dear PARI developers,

We have updated ellrank to compute the Cassels pairing.
As the result the output is now a 4 component vector:
[r,R,s,L] where r, R, and L are as before and s is the 2-rank of
Sha[2]/2Sha[4].

Compared to the previous version we have old_R = new_R + s,
so the new version is more accurate.

For example:
? ellrank(ellinit([0,0,0,-1,351298031616]))
%1 = [3,3,2,[[-1,592704],[1,592704],[-7056,84]]]
instead of 
%1 = [3,5,[[-1,592704],[1,592704],[-7056,84]]]

So now, we know that the rank is exactly 3.

Cheers,
Bill.