Fix treatment of input vectors in conditional_forecast_paths

According to the manual, the (xx) syntax to input arbitary vectors should work for conditional_forecast_paths:

conditional_forecast_paths;

var e;
    periods 1:3;
    values (xx);
end;

But that is not the case, as the code is translated to

constrained_paths_(1,1)=xx;
constrained_paths_(1,2)=xx;
constrained_paths_(1,3)=xx;

in the preprocessor. The mapping from a vector on the right to a scalar and the left leads to crashes. In contrast, the analogous syntax in the shocks or irf_weights block does not suffer from this issue, because here the preprocessor just writes a structure/cell array and the translation is done in Matlab, e.g.

M_.det_shocks = [ M_.det_shocks;
struct('exo_det',false,'exo_id',4,'type','level','periods',1:3,'value',xx) ];

We should harmonize this. My preference is writing a structure in the preprocessor and handling the translation on the Matlab end.