Spectral derivatives of r(theta,phi), gauss-legendre quadrature and various fixes

• Add computation of derivatives of r(theta,phi) from spectral expansion.
• Add support for gauss-legendre collocation points and quadrature.
• Add Ker metric in cartesian coordinates (not tested).
• Add checks on AHF module to work locally without installation.
• Fixes to Kerr-Schild metric and add functionalities (WIP).

added 3 commits

• 0c64f4cf...831fb0ac - 2 commits from branch master
• e7e9db61 - Merge branch 'master' into 'feature/gaussleg'

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mentioned in commit 1907bd64

merged