DFT+U: FFT and Psolver Poisson solvers on submeshes
Description
Implementing two new Poisson solver (FFT and Psolver) for solving the Poisson equation on a submesh. This is used to speed up the calculation of Coulomb integrals in DFT+U calculations. Both of the solvers are used as non-periodic Poisson solver, as this is required for the calculation of Coulomb integrals.
News snippet
Implementing two new Poisson solver for solving the Poisson equation on a submesh.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.