Store info on deflator growth factor in M_ for each endo variables

Assume we define a model with trend variables:

trend_var(growth_factor = exp(GYTREND0)) YTREND0;
trend_var(growth_factor = exp(GP0)) PY0;

// real variable
var (deflator=YTREND0) Y;
// price index
var (deflator=PY0) PY;
// nominal variable
var (deflator=YTREND0*PY0) YN;

It would be extremely useful if we had trace in M_ of the preprocessed trends, e.g. similarly to name/long_name, there could be a cell list M_.endo_growth_factor, which would contain for the above example:

M_.endo_growth_factor(1) = {'exp(GYTREND0)'};
M_.endo_growth_factor(2) = {'exp(GP0)'}
M_.endo_growth_factor(3) = {'exp(GYTREND0)*exp(GP0)'};

for variables not trending it would be:

M_.endo_growth_factor(4) = {'1'};