Improved Performance of Statistical Aggregate Functions

Improved performance of aggregate functions varSamp, varPop, stddevSamp, stddevPop, covarSamp, covarPop, corr by reducing computational stability. The old functions are available under the names varSampStable, varPopStable, stddevSampStable, stddevPopStable, covarSampStable, covarPopStable, corrStable.