allow to associate initial conditions to shocks in shocks grouping for shock decomposition plots

It would be useful to have an interface for a command like init2shocks, which accepts couples of names. first an endo name, second a shock name

init2shocks;
endoname1 exoname1;
endoname2 exoname1;
endoname3 exoname2;
...
;

there can be multiple endo names for one exo, but NOT viceversa.

The preprocessor should translate this into a cell matrix as follows:

M_.init2shocks = {
'endoname1','exoname1';
'endoname2','exoname1';
'endoname3','exoname2';
};

default will be: M_.init2shocks = {};

Then we need a new option init2shocks for plot_shock_decomposition(init2shocks). If it is called, the preprocessor will trigger

options_.plot_shock_decomp.init2shocks = M_.init2shocks;

otherwise it should be empty (by default).

options_.plot_shock_decomp.init2shocks =[];

with this definitions, plot_shock_decompo will attribute the initial condition effect of endo variables to the shock/shock group.