Attempt to find a zero of a functionf with derivative df and second derivative ddf using Halleys's method, returning after two successive values are within epsilon or after maxn tries (then returns null)
Exekutatu Newton-en metodoa polinomio batean erro bat aurkitzen saiatzeko; emaitza ematen du elkarren segidako bi balio epsilonen barruan daudenean, edo maxn saiakera egin ondoren (orduan null ematen du)