Add Anomaly Detection To Fix False "Gas Free" Buildings

[vkoves]

Some buildings report 0 natural gas use in the latest year but have historically reported high levels of gas use. Since it's incredibly unlikely a building all of a sudden goes to 0 (particularly from a quite high number), we should flag these and not show them on our gas free page, or show the positive notice in the Natural Gas Panel.

This could also catch buildings that became vacant and have very dramatic energy use decreases.