Check size of MeanForecast fields and make it consistent across options

oo_.MeanForecast.Mean.variablename and oo_.PointForecast.Mean.variablenam are n+1 vectors if mh_replic>0 and forecast=n, but only n-vectors if mh_replic=0. This should be made consistent or at least documented. See http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=5987