Fix incorrect parameter transformation for X5 in GR5J

In TransfoParam_GR5J.R, the proposed transformation for X5 only allows values between 0 and 1. Indeed, with ParamOut[, 5] <- (ParamIn[, 5] + 9.99) / 19.98, with ParamIn[, 5] between -9.99 and 9.99, we get bounds of 0 and 1.

Yet, X5 can take values below 0 and above 1 (see figure 8 of this article for instance). The proposed transformation in TransfoParam_GR6J.R, which is ParamOut[, 5] <- ParamIn[, 5] / 5, seem to be more consistent with the expected values of the parameter, in addition to improving coherence between the two versions of the model.