Add 'stdev' statistics to PoolAggregator, and use it instead of 'var' by default

[dbogdanov]

The standard deviation is expressed in the same units as the mean is, whereas the variance is expressed in squared units. Therefore, it is more convenient to use standard deviation to describe the variability of data.

Add a new option 'stdev' to compute standard deviation to PoolAggregator. Make this option default instead of 'var'.

Make sure Music Extractor still uses 'var' for compatibility with previous versions of this extractor (and compatibility with AcousticBrainz data) although in future we'd rather switch to 'stdev' as well, or store both 'var' and 'stdev'.