multiple routes - helper functions to group PurpleAir sensors

[tbec]

PurpleAir has who separate sensors in a single housing: "parent" and "child". The child sensor just has PM2.5, while the parent sensor has PM2.5 + other sensors. Tom is adding a column to the BigQuery table called PurpleAirParent. For child sensors, the value is the DeviceID of the parent sensor. The value is NULL for parent sensors.

We'll want some helper functions to combine PurpleAir parent/child sensors. Kerry suggested to use this formula to combine PM2.5 measurements from the two sensors:

# Threshold needs to be adjusted, but somewhere in 30% is probably good
avg = average(parent[PM2.5], child[PM2.5])
if parent[PM2.5] <= 5 and child[PM2.5] <= 5:
    return avg
else if abs(parent[PM2.5] - child[PM2.5]) / avg < THRESHOLD:
    return avg
else:
    return NULL

We should probably just return the parent sensor with this altered PM2.5 value, maybe a field denoting the the child sensor as well:

{
    "Sensor ID": "PP1200",
    "Secondary ID": "PP1201"
}