Integrate doubling algorithm into lyapunov_symm

Following #988 (closed) and the discussions at the 2015 Dynare conference, we should integrate disclyap_fast.m into lyapunov_symm to have one solver only. The question is whether we want to integrate this as a call to disclyap_fast.mas an external function or whether to make disclyap_fast.m a nested function within lyapunov_symm. I would prefer the latter, because it makes the function with the doubling algorithm reusable as a standalone in the way Herbst (2015) and Schmitt-Grohé/Uribe already use it.

When doing this, we also need to discuss the option handling as we currently do not pass the doubling tolerance to lyapunov_symm. Currently, we pass lyapunov_fixed_point_tol. I would propose to use this input argument as the tolerance criterion if the doubling algorithm is selected.