Integration with Jax-XC

[PabloAMC]

Jax-XC allows to convert LibXC functionals to JAX. It would be cool to allow integration with our library, I think.

[jackbaker1001]

I contacted the developers to see the best way to expose the energy densities. Let's wait and see...

[jackbaker1001]

Ok so this should in principle we quite straight-forward. Great find @PabloAMC. I'll make an attempt at this shortly.