v4.1:: Changes related to lyapunov_symm.

+ Fixed a bug introduced in the previous commit.
+ Added a threshold parameter for the complex blocks in the upper
triangular matrix T in lyapunov_symm.m (new field in options_)
+ Cosmetic changes.


git-svn-id: https://www.dynare.org/svn/dynare/trunk@2414 ac1d8469-bf42-47a9-8791-bf33cf982152