When the the variable in which we are saving the current sum is of
integral type we can use a much more optimized apply() method: instead
of looping the entire number of samples adding them up, we always keep
the current sum in a member. It also allows the caller to decide the
type it wants to use to get the average: this may be dictated by another
interface it uses or it may want to avoid the implicit truncate in the
return of apply().
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