Primitive part of a polynomial : primpart

primpart takes as argument a polynomial P given by a symbolic expression or by the list of it's coefficients.
primpart returns the primitive part of P, that is P divided by the GCD (greatest common divisor) of it's coefficients.
Input :

primpart(6x^2-3x+9)

or:

primpart([6,-3,9],x))

Output :

2*x^2-x+3