Overhaul for std

[Jolanrensen]

Helps #961

• Simplifies BasicStats file, used by std and (pearson) correlation for the efficient calculation of count, mean, and variance
• Overhauls the std implementation and public API
• Adds preliminary kdocs for the ddof default and what it means, needs to be expanded later
• Fixed some generics nullability for mean and sum
• Changed rowXOf behavior for min, max, mean, sum, std to go for the value-type instead of the column type