Preprocessor: changes to code for chain rule derivation

* fixed a bug in the handling of VariableNode: we now make a copy of the recursive_variables map, instead of modifying that of the caller
* factorized code shared with standard derivation
* various minor cleanups


git-svn-id: https://www.dynare.org/svn/dynare/trunk@2811 ac1d8469-bf42-47a9-8791-bf33cf982152