reverses roots lower than 1 in a L polynom
[AR,corrv_ar]=transf_roots(AR)
* AR = a (k x 1) vector of coefficients of a polynom 1+AR(L)
* AR = the (k x 1) vector of transforms coefficients such as all roots of (1+AR(L)) are greater than 1
* corrv_ar = the coefficient that lust be used to correct the variance of residuals