adding Proof option to factor()

["John Cremona" <john.cremona@gmail.com>]

The factor() function for integers (i.e. factorint()) does not provide
factors which are proved to be prime, unless they are are < 10^15,
accoriding to the manual.  The manual recommends using isprime() in
circumstances where proven primes are required.

Could we add another flag to the function which defaults to 0 but if
set does the proving step too?  I use pari's factor() in mwrank and am
not happy that I cannot guarantee results because of this, and it
would be a lot easier to just add af lag to the call I make to
factor().

-- 
John Cremona