Note about libxc/libnxc unpolarized functional

In a libxc or libnxc correlation functional, the current dftsw implementation always uses the functional in spin polarized form in calculations.

In the case of spin unpolarized systems with GGA, XC_POLARIZED Ec[\(\rho_\uparrow\), \(\rho_\downarrow\), \(\sigma_{\uparrow\uparrow}\), \(\sigma_{\uparrow\downarrow}\), \(\sigma_{\downarrow\downarrow}\)] (where \(\rho_\downarrow = \rho_\uparrow\)) is equivalent of XC_UNPOLARIZED Ec[\(\rho_{tot}\), \(\sigma_{tot}\)].

\(\rho_{tot}\) is twice the \(\rho_\uparrow\) and \(\sigma_{tot}\) is four times the \(\sigma_{\uparrow\uparrow}\).
For meta-GGAs, add \(\tau_{tot} = 2 \times \tau_\uparrow\) in Ec[\(\rho_{tot}\),\(\sigma_{tot}\),\(\tau_{tot}\)].