irf0 — 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