Allow leads lags in # declarations

currently, if one wants to define auxiliary terms to be replaced in the model, appearing at different timing, one should define them as separate objects:

#du_dc = c^(-TAU);
#dup_dcp = c(+1)^(-TAU);

it would be handy to allow using leads and lags of the lhs terms in the model, i.e.

du_dc(+1) would be handled by the preprocessor by leading all endo variables on rhs: c(+1)^(-TAU)