Print out Libxc citation and version information

[susilehtola]

Johannes Neugebauer contacted me about the use of Libxc in Serenity, but it appears that the use of Libxc or the employed functionals is not documented to the users of Serenity.

The Libxc interface should print out

• that Libxc is used, and the employed version (xc_version_string())
• the up-to-date literature reference of Libxc that should be cited in any paper employing a Libxc implementation of a density functional (xc_reference() and xc_reference_doi())
• references to the original literature describing the density functional (xc_func_info_get_references() to get individual references and xc_func_reference_get_ref() and xc_func_reference_get_doi() to get the reference's information)

These points should also be documented in the manual (I did not check).

This information is important not just to keep Libxc rolling (if nobody cites it like now funders think it's not important), but also for reproducibility: different implementations of a given functional may give incomparable results, see our recent discussion in

S. Lehtola and M. A. L. Marques, Reproducibility of density functional approximations: how new functionals should be reported, J. Chem. Phys. 159, 114116 (2023). doi:10.1063/5.0167763

[TalpaE]

Thank you for noting this; it has brought the missing references to other external libraries to our attention as well. We will fix this shortly.