Fix computation decision rules for deterministic variables

For the mod-file reported in #711 (closed) from http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=5951 the decision rules are different from the 4.2 branch starting in the 4.3 branch. In particular, dr.ghudud differs.

When fixing this, we should also add information on how the decision rules are actually computed to the manual and potentially add a unit test.