Correctly implements a multigrid Poisson solver

Nicolas Tancogne-Dejeanrequested to merge
multigrid_poisson_solver into main

Description

Correctly implements a multigrid Poisson solver

As expected, a 4-level V cycle shows a clear speed-up compared to the current (wrong) implementation.

The multigrid Poisson solver can now also be used with curvilinear meshes.

News snippet

Re-implement the multigrid Poisson solver. It can now be used with curvilinear meshes too.

Checklist

  • I have checked that my code follows the Octopus coding standards
  • I have added tests for all the new features added in this request.
Edited by Nicolas Tancogne-Dejean

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