perf: chan's parallel mean-var algorithm for dask#4123
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See https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Parallel_algorithm
Based on a discussion with @zboldyga
This has two benefits - it allows us to calculate mean/var in one pass instead of effectively two (square sum and sum squared) and gets rid of a numerical instability issue that @zboldyga found the solution to (see removed comment)