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Implement Chebyshev expansion for phi_1 and phi_2

Sebastian Ohlmannrequested to merge
chebyshev_phi_functions into main

Description

For the expgauss schemes for Maxwell's equations, evaluations of \phi_1 and \phi_2 of the Hamiltonian are needed. This MR implements those for Taylor and Chebyshev expansions (Lanczos has been implemented before).

For the Chebyshev expansion, compute the expansion coefficients using a discrete cosine transform from FFTW. Also refactor the coefficient and error computation into a separate module.

With these changes, the Chebyshev expansion can be used for the expgauss schemes for Maxwell. Corresponding tests are added.

News snippet

Enable Chebyshev expansion for expgauss schemes for Maxwell

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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