Make `GeneralizedEigenScalar` and `SymmetricEigenScalar` from `nalgebra_lapack` public

[Devoev]

The traits nalgebra_lapack::generalized_eigenvalues::GeneralizedEigenScalar and nalgebra_lapack::symmetric_eigen::SymmetricEigenScalar are private, while the other special scalar traits are public.
This makes it impossible to write generic code over generalized or symmetric EV decompositions. Is there a good reason why these traits are private, while the others are public?