Integrate CEMS data for units that only report NOx

[grgmiller]

A handful of units that report to CEMS only report NOx emissions data, but report zero data for gross load, fuel consumption, and other pollutants. As of https://github.com/singularity-energy/hourly-egrid/pull/133, we now remove these observations from the dataset in favor of using EIA-923 data.

However, in the future, it may be worth adding this standalone hourly NOx data back into the results, as it could help us shape the EIA data.