Print out citation information from libxc

[susilehtola]

Hi,

DFT-FE appears to use libxc exclusively to evaluate density functionals, but doesn't seem to be printing out any information about it. I would kindly ask to include the following information in the DFT-FE output:

• the version of libxc used (produced by xc_version_string())
• the citation for the used version of libxc (journal reference produced by xc_reference() and doi by xc_reference_doi())

In addition to Libxc, users should also cite the employed density functionals. This information is conveniently stored inside Libxc in the func->info->refs field, which should be likewise printed out by DFT-FE.