On-line mean and variance estimates for a stream of fractional values. Uses
Type S must have a scala.math.Fractional[S] witness in scope, a
NumericLimits[S] witness in scope, and a function converting from
Long to S in scope. Scala provides the former and the latter for its
fractional numeric types; the SampleSink companion object provides witnesses
forNumericLimits[Float] and NumericLimits[Double].