Remove support for Libxc 2
This removes support for Libxc 2. The last bugfix release of Libxc 2 is four years old now and Libxc 5 will be released soon. Also mark Libxc 3 as deprecated, as Libxc 4 it's already two years and a half old.
Remove support for Libxc 2 and mark Libxc 3 as deprecated.
- 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 #269 (closed)