Add support for V2 fans (those that report PM2.5, PM10, and NOx)

This adds a bunch of new metrics specific to V2 fans (described in
README). Effort was put into maintaining compatibility between both
models as much as possible, e.g; VOC is scaled from V2 fans down to
the original range from V1 fans (0-100 to 0-10 respectivey). Additionally,
some metrics only available on one were synthesised for the other,
e.g; fan_mode, fan_power, and auto_mode.

Also: run pyformat on the files.