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Fix computation of stencil weights for non-orthogonal cells

Sebastian Ohlmannrequested to merge
fix_stencil into main

Description

Always compute the polynomials that are used to generate the weights in primitive coordinates and compute the right-hand side taking the metric into account. Moreover, ensure that the star_general stencil uses a set of polynomials that yields a non-singular matrix for all 14 Bravais lattices. This removes the need for using the singular value decomposition.

With these changes, both the default (i.e. star_general) and the cube stencil give reasonable errors for the Laplacian and the gradient on all 14 Bravais lattices. Tests are added for all these cases.

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Fix computation of stencil weights for non-orthogonal cells

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.

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