Rework `getMean...()` functions

[Gregorio Berselli]

We need at least two overloads:

• getMean...(double threshold, bool congested = true) for the congested/uncongested streets. i.e. streets with a density higher/lower wrt a certain value;
• getMean...(Id streetId) to get the mean value over a street
• getMean...(Size nStreets) to get the mean value over nStreets random chosen streets