Implement new "averages" endpoint

[lionralfs]

For the frontend, we need an endpoint which gives us an average for an entire day/hour(?) per sensor.

Ideally, the frontend requests the data for a single day/hour and the endpoint returns a list of sensors which include the average measurements for that sensor grouped into 5min intervals.

For example, the result could be a map <sensor_id> --> list of 288 values (1440 minutes per day / 5 minutes = 288)

{
  "92": [ { "p1": 4.2, "p2": 3.5 }, { "p1": 5.2, "p2": 3.3 }, "286 more...." ]
}