Added samplestandarddeviation and sstddev as synonyms for stddev; internal changes to better identify stddev as _sample_ standard deviation (in preparation for the introduction of _population_ standard deviation)
Added population standard deviation function (populationstandarddeviation/psdddev)
Added average deviation function (averagedeviation/avgdev)
Added arithemeticmean and mean as synonyms for average; internal changes to better identify average as _arithmetic_ mean (in preparation for the introduction of _geometric_ and _harmonic_ mean)
Added geometric mean function (geometricmean)
Added harmonic mean function (harmonicmean)
Added sample and population variance; applied some tweaks for clarity.
Added separate parse and execution errors for new functions
Fixed avgDev execution error.
Revised shorthand synonyms to minimise chance of clashes with variable names
Fixed shorthand synonyms alphabetical order
Another alphabetical order fix
Added release notes
Merge branch 'feature-new_statistical_functions' of https://github.com/quartamsoftware/livecode into feature-stats_functions
[[ StatsFunctions ]] Added referencing comments.
[[ StatsFunctions ]] Tweak to notes to fix formatting.
Merge branch 'feature-stats_functions' into develop