This function calculates a standard deviation of values as an exponential series using a constant weighting factor represented as a bar count.
As in the EMA function, count is converted to a weight factor using the formula factor = 2 / (count + 1).
This factor is then used to calculate two intermediate series for an array of values.
An EMA is calculated in place of the mean in a typical StdDev calculation.
The average of the squared differences between each value and the mean (i.e. the standard deviation) is also calculated as an EMA using the same weight factor.
