Use PM2.5 instead of composite AQI in calculating fresh air paths and air pollution exposure (?)

[hellej]

In the composite AQI, the index value is always set by the worst of the pollutants. Hence, there may not be any variation in AQI if one (spatially non-varying) pollutant "overrides" the index. In that situation, the user could actually benefit more from routes optimized by a non-composite AQI (e.g. by just PM2.5).