Allow for no_graph option in plot_shock_decomposition

It would be useful for me to allow the option no_graph to plot_shock_decomposition. It seems a bit silly, but one may be just interested in the xls output without the fig one.

The option would trigger

options_.no_graph.plot_shock_decomposition=1

while by default it will be

options_.no_graph.plot_shock_decomposition=0

in global_initialization