Add support for Libxc 5
This MR adds support for the upcoming Libxc 5. To make this simpler to implement and maintain, now Octopus uses the Fortran 2003 interface of Libxc for all versions.
Add support for upcoming Libxc 5.
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.
Closes #287 (closed)