predictAR lookback abstraction

These changes abstract out the lookback interval from the predictAR function. Currently predictAR always uses the last data point (5 minutes ago).  This change allows us to set lookback=2 to use the average slope for the last 10 minutes as input into the AR function.   This doesn't actually change the actual predictAR behavior yet (because I'm leaving lookback=1 for now): that will be a follow-up pull request to set lookback=2.

The one thing this does change is the behavior when brushing onto missing retrospective data.  Previously, it would display predictions from the last data point, and display that data point numerically at the top.  This change has the side effect of causing the prediction to disappear when brushing over missing data, and display "---" at the top instead of stale data.  I think this is actually the more correct behavior, so I haven't tried to recreate the original behavior in the new code.

This change does not affect the behavior of late-arriving live data: it still displays a cone from the last data point for awhile longer, as before.