* reshaped DataTree/ModelTree using a more object-oriented approach
* improved expression sharing algorithm (temporary terms in output files), resulting in smaller output files for big models
* optimized performance (both in computing time and memory consumption)
* bugfix: the expressions stored in local parameters of models (pound expressions) are now derived
* bugfix: cosh(0) = 1
* removed some useless parentheses in output


git-svn-id: https://www.dynare.org/svn/dynare/dynare_v4@1148 ac1d8469-bf42-47a9-8791-bf33cf982152