new item to add to the running average
amount by which to change a datum in the running average
standard deviation of data
a (possibly immutable) object whose average is the negative of this object's
item to remove from the running average
if count is 0
Extends
FullRunningAverage
to add a running standard deviation computation. Uses Welford's method, as described at http://www.johndcook.com/standard_deviation.htmlPorted from org.apache.mahout.cf.taste.impl.common.FullRunningAverageAndStdDev.java