Habilitating libvdwxc interface

A few things are missing in order for the libvdwxc interface to no longer be marked as experimental:

• Install libvdwxc on all the test farm machines
• Commit automatic unit tests: molecule, solid with non-trivial lattice vectors
• Make sure documentation is up to date
• Proper options for ./configure
• Accurate verification of forces/energy

Also it would be nice to

• Add spin-polarized vdW formalism (provided by libvdwxc, but we still assume spinpaired on the Octopus side)
• Add appropriate test

Perhaps @micael.oliveira might want to add to this list.