(high-priority) Don't store PPMs for sensors that are maxxed out.

Significant issue for the risk log...

**The Issue**
* The current NO2 sensor has a range of 10ppm. Ideally, Prometeo firefighters want the limit to be set at 20ppm, but since 20ppm isn’t possible with this sensor, a 10ppm limit has been proposed. Unfortunately, that can't be done safely.
* An example to illustrate:
* A firefighter experiences [30mins at 1ppm. Then 30mins at 25ppm]
* One hour into the fire, this firefighter has experienced an average of 13ppm per hour, well _over_ the 10ppm limit - their status should be ‘Red’.
* The command center would see their status as Green (not Red or Yellow) with a 5.5ppm per hour average - seemingly well _under_ the limit, because the sensor provides [30mins at 1ppm. Then 30mins at 10ppm] (averages to 5.5ppm)
* For time-weighted average math to give reliable results, the sensor must have a much larger range than the limits. If the limit is set at or near the sensor range, a firefighter's status would essentially never go red, even if they were well over the limit. Not a viable solution.

Options
1. Use sensor with a bigger range - this is in motion, but won't be available for a while.
2. While waiting for #1, have a way to indicate when sensors are maxxed out and store that rather than a ppm reading. Also make the analytics engine stop generating TWA results for any time-windows affected by maxxed-out sensor readings.
3. It's not yet known if there there will be time to implemnent #2 before the first events, a safe fallback is needed. That fallback is to record the NO2 values during these events, but not to generate analytics for them on the dashboard. This is a one-line configuration change to prometeo_config.json - to remove nitrogen dioxide from the 'supported gases' configuration property. Once a 'maxxed out detection' is implemented, this NO2 configuration can be switched back on.

This github issue (#117) is to track option #2. Option #3 has been implemented and is pending merge ([pull request #9](https://github.com/Code-and-Response/Prometeo-Rules-Decision/pull/9)).