Clean up leaky interface that requires discrete_time_problem to know about time freezing.

[apozharski]

Currently Stewart and Heaviside problems use Options.time_freezing to identify a physical time state. This makes the resulting implementation confusing. A possible solution to this is just adding a new option that specifies the existence of a physical time state.

[nurkanovic]

I agree. And the default is of course that the clock state does not exist.