Enable setting t in NonlinearSystem creation

[TorkelE]

We should have an option for setting the value of t when nonlinear systems are created. Next, e.g Bifurcationproblem and hc_steady_states could use this functionality to permit the user to set t for which the steady states are found (probably t = Inf).

Once #739 is merged, we have a is_autonomous check in these cases (which yields an error if they are not).