Implement a batchified version of the multigrid preconditioner.
Description
Implement a batchified version of the multigrid preconditioner. This makes the combination RMMDIIS + multigrid faster. A test is added to cover this combination. This also fixes a bug for non-orthogonal cells with the multigrid preconditioner.
News snippet
Optimization of the multigrid preconditioner for RMMDIIS.
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.
Edited by Nicolas Tancogne-Dejean