* created a distinct expression tree for the static model (thus giving better sharing of sub-expressions and better computation of temporary terms for the static model)
* for that purpose, created StaticModel and DynamicModel classes (ModelTree still persists, but only contains code shared between StaticModel and DynamicModel)
* removed sparse static file (to be later replaced by new algorithm for steady state computation on large models)


git-svn-id: https://www.dynare.org/svn/dynare/trunk@2592 ac1d8469-bf42-47a9-8791-bf33cf982152