private Cholesky decomposition, which has not unittests yet. PRs are welcome.
My fork of lubeck has a branch where I was doing some work on adding cholesky that has some unittests if you want to borrow them (probably needs some adjustments).
https://github.com/jmh530/lubeck/blob/f012582871c0cde8d7dc18d7f833f513a06cfaca/source/lubeck.d#L1377