Integrate functionality to generate density matrices for LDA+U
@mlezaic has a script to generate a density matrix for LDA+U. We should make this functionality available to all Fleur users. To me it is not yet clear in what way, whether we keep it as an external script or whether we should integrate it into Fleur or into an AiiDA workflow or so. But as a starting point the script (for an older Fleur version) is available.