RSSI median or table to allow for better interpretation of data

To allow better interpretation of pax values or (manual or semi automatic) adjustment of RSSI thresholds libpax should provide data about the distribution of RSSI values over the counted devices.

- for low bandwidth (e.g. LoRaWAN) uplinks a median value of the count cycle RSSI values would be sufficient (could also be left up to the framework outside libpax)
- for high bandwidth links (e.g. ethernet/MQTT) a table/matrix of RSSI value / counted devices can provide a better insight

The feature should be runtime configurable, as it may use a significant amount of CPU and/or memory. Aggregation options like granularity might also help to reduce memory and bandwidth when dealing with tables, however make data interpretation more complex after changes. Nevertheless, this would be an option for low bandwidth devices to transmit tables.