Low-level Impulse Response function
[IRF,PHI]=irf0(beta,S,N,p,P)
* beta = estimated parameters from a VAR
* S = # of periods
* N = dimension of the VAR
* p = # of lags
* P = matrix such that P*e = u where u is the residual from the VAR regression; e is the residual to be shocked
* IRF = ((S+1) x T) impulse response functions
* PHI = (N*p x T) matrix of coefficients