utilities/python_influxdb_downsample#26: Add support for derivative and difference

Some stats in the database are incrementing counters - network interface stats often use these - so rather than using the raw figure, it's often desirable to calculate a delta, or even the rate of change (i.e. the delta divided by time elapsed).

Using these in Flux is pretty straightforward:

|> difference(nonNegative: true)
|> derivative(nonNegative: true)

We don't currently provide an easy means to have these applied to a query though. We should either add them as some sort of a pre-processor (i.e. have downsample calculate it) or add a means to have it injected into the Flux query.