galoisinit for large nilpotent groups

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

Dear PARI developers,

I have pushed to master my new algorithm for galoisinit for nilpotent
groups. The function should detect automatically the group is nilpotent
automatically.

For nilpotent group of order larger than 100, this should be much faster
than the previous algorithm.

Cheers,
Bill.