This private function allows to compute scaling constants in the valuation formulas.
computeConstant(id, scale, cov.matrix)
an integer value. The risk-factor ids involved in the valuation formula.
a numeric value. The scales corresponding to those risk-factors.
a numeric matrix. The covariance matrix of the risk-factors.
A numeric value, the scaling constant. This is equal to -0.5
times the variance of the linear combination of the risk-factors provided
in the parameters.