introduce posterior_nograph option

I think it would be useful to have an option like:

options_.posterior_nograph

with default value at 0 like standard nograph.

This option would allow performing all possible posterior subdraws for irfs, smoothed variables etc, but avoiding to produce thousands of useless graphs that are currently done [for large models at least].

I think it would also be useful to make a distinction w.r.t. options_.nograph since the latter would not trigger prior/posterior plots of parameter estimates which would be required in any case.

And by the way, pm3 and posterior_irf do not seem to honor the nograph option: plots are made in any case. So at least we should make sure nograph would work properly?

If you agree, I would make the appropriate changes to the matlab pm3 and posterior_irf routines with posterior_nograph [the preprocessor could be updated with this extra option, then].