Add support for Graphite web holtWintersConfidenceArea() function

[carrieedwards]

This PR adds support for the Graphite web holtWintersConfidenceArea() function.

The holtWintersConfidenceArea() function is defined as:

holtWintersConfidenceArea(seriesList, delta=3, bootstrapInterval='7d', seasonality='1d')
Performs a Holt-Winters forecast using the series as input data and plots the area between the upper and lower bands of the predicted forecast deviations.

[CLAassistant]

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3 out of 4 committers have signed the CLA.

✅ npazosmendez
✅ leizor
✅ carrieedwards
❌ msaf1980
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