Draft: Neuralxc
Adds interface for the NeuralXC "functional", which relies on an ML approach (see https://github.com/semodi/neuralxc). The skeleton of this interface has been kindly provided by Marivi Fernandez-Serra's group.
Testing still needs to be done. Also, it relies on ForPy, which basically calls python from fortran and I'm not really sure I like it.
Also:
- Need to improve the repo fetching for ForPy.
- There could be a way to provide the proper python linking without relying on SIESTA_LINKER_FLAGS_POST...