fix float64 series use mean or stdev function result is zero

[anywhy]

series := types.NewFloat64Series()
	series.PushAndEmit(2)
	series.PushAndEmit(4)
	series.PushAndEmit(6)
	series.PushAndEmit(8)
	series.PushAndEmit(10)

	t.Log(series.SeriesBase.Length())
	t.Log(series.Mean(3))
	t.Log(series.Stdev(3))
}

The result is zero.

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[zenixls2]

can you help adding some unit test cases on this struct?

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Re-estimated karma: this pull request may get 238 BBG