Estimate numerically the variance-covariance matrix from the GVAR-based models
EstimationSigma_GVARrest(SigmaUnres, res, IdxVarRest)
restricted version of the variance-covariance matrix a GVAR model (K x K)
Unrestricted variance-covariance matrix (K x K)
residuals from the VAR of a GVAR model (K x T)
index of the variable that is selected as strictly exogenous