branch bill-bnfinit-parallel

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

Dear PARI developers,

I have pushed a branch bill-bnfinit-parallel which an attempt to merge the parallel bnfinit code
to master.

The branch does not affect bnfinit unless the tech parameter is used.
It adds 2 extra options:
instead of [c_{1},  c_{2},  nrpid ], no the full vector is
[c_{1},  c_{2},  nrpid, maxfact, idex,  nbthr ]

-- maxfact is the maximal number of factorizations to try by ideal.
   (500 by default)

-- idex is the exponent of the ideal to use when searching relation.
   The default is 0 which lets PARI pick an exponent depending on the norm of
   the ideal.

-- nbthr is the number of threads to use (1 by default). If set to 0, use default(nbthreads).
When nbthr is not 1, the parallel bnfinit code is used.

One example when the tuning make a large difference:

x^28+1154*x^26+489873*x^24+100211629*x^22+11699125060*x^20+854076746718*x^18+40959569036260*x^16+1321233290968720*x^14+28785594434947700*x^12+417246803388550268*x^10+3867079700626715505*x^8+21181990617517025086*x^6+59006973863083141311*x^4+63878093656658485059*x^2+21292697885552828353

Cheers,
Bill.