Support explicit functions of time in the equations of motion

[moorepants]

opty fails if you pass it an equation like: $\dot{v}(t) = \sin(t)$ because time is explicit in the sin term. We have a hack which is to introduce a variable for time $\dot{v}(t) = \sin(\tau(t))$ where $\tau = t$, but it would be better if that hack was not required. Instance constraints should also support explicit functions of time.