Fix charging-stats for better "Charge deltas" when the charging process is interrupted and re-started

[nicoladefranceschi]

When the charging process is interrupted and re-started, you can see some "jumps" in the graph.
Right now the graph looks like this:

With the new query, this is the result:

To achieve this, the query is modified to aggregate charges within a few hours

SELECT
	$__timeGroup(start_date,'2h') as time,
	MIN(start_battery_level) as start_battery_level,
	MAX(end_battery_level) as end_battery_level
FROM
	charging_processes
WHERE
	$__timeFilter(start_date)
	AND duration_min > 3
	AND car_id = $car_id
GROUP BY 
	1
ORDER BY
	1;

[adriankumpf]

Thanks!