moved time step adaptivity algorithm to a separate routine, to have one spot to modify for all solvers.
moved time step adaptivity algorithm to a separate routine, to have one spot to modify for all solvers.

liuqx315/Sundials,liuqx315/Sundials,liuqx315/Sundials,liuqx315/Sundials,liuqx315/Sundials,liuqx315/Sundials