First stable v3 release:
[Upgrade Notes](https://github.com/mathnet/mathnet-numerics/wiki/Upgrading-to-Version-3)
Stable API, no more breaking changes for all future v3 releases (except previews).
Finally unlocks development and contributions around non-linear optimization and native providers over the next few minor releases.
Native Providers: option to control max number of threads used by MKL.
F#: Fit.multiDim; Matrix.qr, svd, eigen, lu and cholesky.