init X for armax/expuh is limited to 1 time-step

This is suitable for n=1, and doesn't matter if warmup>0

armax.sim works fine, but hydromad treats length(init)>1 as an unspecified parameter.

armax.sim(U=c(1,rep(0,20)),a_1=0.1,a_2=0.1,b_0=1,init=as.numeric(Murrindindi$Q[1:2]))
armax.sim(U=c(1,rep(0,20)),a_1=0.1,a_2=0.1,b_0=1,init=as.numeric(Murrindindi$Q[1]))
