Automatically create equation name tags for auxiliary equations

By default, Dynare creates equation name tags for the original equations of the model block. For example, if

rn = r + rho*(rn(-1)-r) + e;

then M_.equation_tags will be

    {[7]}    {'name'}    {'rn'}

But nothing is created for auxiliary equations. We should at least change this for ramsey_model where the first equations will correspond to the derivatives with respect to the endogenous variables. For example, if M_.endo_names{1}='y', then M_.equation_tags should have as its first entry:

    {[1]}    {'name'}    {'Ramsey FOC wrt y'}