fix error in state_at history scaling logic

@vreeken This addresses some annoying behavior from the state_at() method. Currently, if we set scaled=False and t < t0, the output is scaled in the wrong direction. We are obliged to set scaled=True to get any usable value, and there is no way to get the correct scaled=True behaviour.

What do you think? In any case, this could be considered a breaking API change.