Refactor BME680 to add VOC index

[tyeth]

The BME680 presents both gas resistance in ohms, and VOC index. As part of adding #103 a voc-index has been added to the adafruit Sensor types. We should refactor the BME680 to include that too.
See #103 (comment)

It's worth noting there is a totalVOC or tvoc sensor type with ppb (presumably Parts Per Billion), but no components yet supporting it. I was under the assumption (from literature mentioning a true voc sensor) that the BME680 presented VOC concentration in an SI unit, but it does not.