Add start and end elevation to drives

[pkuehnel]

Background is e.g. the efficiency Dashboard, which shows depending on temperature the car's efficiency. I drive every day in the morning (low temperature) downhill to work (so lower consumption even though the temperature is lower) and in the afternoon (high temperature) I drive uphill home. Would be cool if we would have the option to add elevation downhill and uphill driven for each temperature in the efficiency dashboard.