Linear regression implementation.

I pushed some commits with changes.

Notable changes:

Use consistent spelling in documentation comments and precondition messages.
Change helper functions to be generic.
Refactor func matrixPseudoInverse into a computed property Tensor.pseudoinverse, which is more idiomatic.

@param087: could you please review the changes to see if they make sense?

I saw that many PRs depend on Helpers.swift - could you please open a separate PR containing just Helpers.swift? Let's try to merge that PR ASAP to unblock your progress.

I have some more feedback regarding LinearRegression.swift, commenting soon.

param087 / swiftML

Linear regression implementation. #5

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