make model runs end at the time specified.

[amoodie]

Currently, model runs specified with the time input to the high-level api do not end at the exact specified time. They get close, but it is inexact because the timestep is not necessarily a factor of the specified end time. One way to address this:

Just before entering the iteration-updating-while-loop, take the model calculated dt = deltamodel.dt and do whole = endtime // dt and rem = endtime % dt, then just set up a loop that runs whole times, and then a single update that sets deltamodel.dt = rem / deltamodel.dt and then runs one final deltamodel.update().