NewCovarianceMatrix, simplify and use Eigen

• Use Eigen operations

• It should be a bit faster.

• But makes also the code a bit simpler and easier to read .

• A quick test https://godbolt.org/z/G48xbP

• New math alone https://godbolt.org/z/jxofxj

• Old math alone https://godbolt.org/z/eo3oT6

I have seen changes at the exact binary level, kind of expected given a slightly different order of operations between doing things 100% explicit and calling Eigen method.

An example from my local tests :

019.GSFConversionVerticesAux.covariance.1.40 -1.1598064020290622e-06 -> -1.1598065157158999e-06 => diff= [-0.00000245%]

So I expect some minor changes

Mentiong @npetters @goblirsc , @amorley , @ssnyder