Add interface for flexible IRF-generation

Create block

generate_irfs(options_list);
(groupname1), exo_name1=1, exo_name 2=-0.5;
(groupname2), exo_name1=2, exo_name 3=-0.5;
end;

or alternatively (as suggested in #115 )

generate_irfs(options_list);
[ name='groupname1' ] exo_name1=1, exo_name 2=-0.5;
[ name='groupname1' ] exo_name1=2, exo_name 3=-0.5;
end;

where options_list can be (for now)

• stderr_multiples translating to options_.irf_opt.stderr_multiples
• diagonal_only translating to options_.irf_opt.diagonal_only

and where each line translates into

1. a cell array options_.irf_opt.irf_shock_graphtitles storing the group_name along the rows
2. a column of a matrix options_.irf_opt.irf_shocks of size M_.exo_nbr*n_lines where non-specified var_exo get a 0 entry.

If no line is provided, leave those fields empty.

The block translates into

1. setting of options_
2. a call to oo_.irfs=generate_irfs(M_,options_,oo_)