Computes the partial derivatives in a VARMA model
[F,dF,A,dA,V,dV,G,dG]=arma_dv(theta,theta2mat,di)
* theta = (npx1) vector of starting values for the parameters
* theta2mat = a string vector of instructions that transforms theta into the matrices FR, FS, AR, AS, V and G
* di = a scalar
* F = the whole AR part of the model
* A = the whole MA part of the model
* V = the variance of the residuals
* G = the coefficients of the exogenous variables
* dF, dA, dV, dG = the derivatives of these matrices with respect to the i-th parameter in theta