Harmonize initval-file loading and update documentation

oo_.exo_simul is periods by exo_nbr, i.e. a collection of column vectors. In contrast, oo_.endo_simul is endo_nbr by periods, i.e. a collection of row vectors. This is confusing and leads to problems with initval-files where this needs to be taken into account. See http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=6261. This should be harmonized in 4.5. Moreover, in initvalf we should simply transpose non-conforming vectors to be more flexibe.