http://xyst.i2p/DEBUG/cppcheck/187.html
for ensemble averaging: (Y-\<Y\>) and (U-\<U\>), then the central moment is //! \<yu\> = <(Y-\<Y\>)(U-\<U\>)>, another example is the third mixed central //! moment of three scalars which needs three terms for ensemble averaging: //! (Y1-\<Y1\>), (Y2-\<Y2\>), and (Y3-\<Y3\>), then the central moment is //!