Better battery calcs

Some changes to how battery time-remaining is calculated under fluctuating readings.

* First, use low-pass filtered voltage for the calculations
* Second, do not rely on the first voltage reading above -1.0V as being definitive.  In practice I sometimes see that initial reading be way low, leading to screwy calculations.  Instead, now any time I see a higher voltage, we reset startVoltage.  As I'm typing this note, I'm thinking that actually you could potentially get a really big spiky high voltage, which would screw things up too.  But this is I think at least better than the current system.  Ideally actually we'd keep a rolling window to do better estimation based on recent power consumption, rather than estimate future power consumption based on all of history, which history likely includes a power-sucking takeoff which won't actually likely recur during the rest of the flight...