Temp-function for computing the full AICc with several X's etc
aicc_full_single_cpp(X, mcov, S_scale_dist, h, y)
matrix with "covariates"
covariance matrix
logical indicating whether the Mahalanobis distance should be scaled with the number of variables
numeric specifying the scaling (sigma)
vector with the "response variable"
Scalar with the numeric value of the AICc formula