* Add computation of derivatives of r(theta,phi) from spectral expansion.
* Add support for gauss-legendre collocation points and quadrature.
* Add Kerr 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).

First working version handling octant and bitant input data symmetries and Athena++ input data reader.

First working version, checked against Kerr-Schild data.