SIESTA-LINRES

Implementation of DFPT within SIESTA. As opposed to initial LINRES project, DFPT will not be a post-processing tool, but will be performed within the siesta run.
-First, standard DFT ground-state calculation with SCF loop in SIESTA, to determined rho0
- Secondly, apply perturbation that correspond to a displacement of a specific atom (the 3 spatial directions are computed on the fly). For each "atomic displacement":
   * SCF loop that determines: (1) perturbed Hamiltonian $\delta H$, (2) perturbed density matrix ($\delta\rho_{\mu,\nu}$ solving the Sternheimer-equation.
   * From $\delta\rho^{SCF}$" one can compute a whole row of the Dynamical Matrix.

Initially, only perturbation that correspond to atomic displacements. Other perturbations might be considered, but "atomic displacements" are more difficult because they include Pulay-like corrections.