[X] Take a list of sensor IDs and try to figure out what's most true,
[X] Pull stats from both sensors when a node has two
[X] Write some documentation
[ ] Write more documentation
[ ] Write more unit tests
[ ] save some state and perform time averaging, perhaps throw out dramatic changes and reuse last known value

[ ] Pull down and cache the all-sensors blob
[ ] ++ find sensors by name
[ ] ++ find sensors nearest a latitude/longitude, or near another sensor

[X] figure out what the metric is, exactly, on the PurpleAir webpage map because it isn't in the api's json blob
[X] ++ improve conversion -- it's the US EPA PM2.5 metric on the map vs raw concentration from the API.
