Relax eigenvector test slightly
Provide inv and det of Tridiagonal and SymTridiagonal matrices
Update NEWS.md
Consolidate det and inv for Tridiagonal and SymTridiagonal

Also specialize det(Tridiagonal) method which calls LAPACK.gttrf!
Restore tridiagonal LU factorization for Rational
Merge pull request #5358 from JuliaLang/cjh/tridiag-invdet

Provide inv and det of Tridiagonal and SymTridiagonal matrices