Feature: new statistical functions as described on the engine forum

Added new statistical functions:
- averageDeviation (aka: avgDev)
- geometcirMean
- harmonicMean
- populationStandardDeviation (aka: pStdDev)
- populationVariance (aka: pVariance)
- sampleVariance (aka: sVariance, variance)
Added synonyms to existing functions:
- arithmeticMean, mean -> average
- sampleStandardDeviation, sStdDev -> stdDev