Local solver: Compute density gradient and Hessian in one go
Previously, computing the gradient and the Hesse matrix of the energy density where two separate operations. For the spectrally split density this was wasteful, because each call involved an expensive eigenvector decomposition. This commit introduces a new method called 'gradientHessian' which returns both first and second derivatives together. That way, the eigendecomposition is done only once, and run-time is saved.