Relax check for varobs statement with measurement error.

The following mod-file leads to a preprocessor error of ERROR: test_ME_shocks.mod: line 18, cols 1-16: shocks: standard error can only be specified for exogenous or observed endogenous variables

var c, k;
varexo e;

model;
c=0.5*c(+1)+e;
k = 0.9*k(-1);
end;

shocks;
var e; stderr 0.009;
var k; stderr 1;
end;

varobs k;

stoch_simul(periods=500);

The reason is that the varobs statement comes after the shocks block. Moving it before the shocks makes the file run. We might want to relax this restriction on the positioning for the check.