Add julia ldlt factorization and solver for SymTridiagonal. Add julia lu factorization for Tridiagonal. Parametrize LU on matrix type. Make pivoting optional in lu. Implement convert(AbstractArray,x) and convert(Factorization,x) methods.
Merge pull request #6265 from JuliaLang/anj/symtri

Add julia ldlt factorization and solver for SymTridiagonal and julia lu factorization for Tridiagonal. Make pivoting optional in LU.