garch_grad2 — Generates garch gradient
[g,dht,gradt]=garch_grad2(parm,nar,nma,y,x)
parm= vector of parameters (beta, a0, ar and ma in that order)
nar = # of ar parameters
nma = # of ma parameters
y = (n x 1) vector of the endogenous variable
x = (n x k) vector of the exogenous variables
g = (k+nar+nma+1 x 1) -gradient at param
dht = (nobs x nar+nma+1) derivative of sigt w.r.t a0, ar, ma
scores = (k+nar+nma+1 x 1) sub-gradient at each date