Two fixes regarding solution of lyapunov equations

• Prevents infinite loops in disclyap_fast as discusses in #821 (closed); also adds error flag for this function
• Makes input arguments of lyapunov_symm unique to fix bug noted in https://github.com/tholden/dynare/commit/69daaa0460b0ddee97292c39d40355201e316622

Still to be adapted to new additional input argument: solve_model_for_online_filter.m from particles submodule