Updated PPACF so that it works with QE V6.4
Some changes are made so that PPACF works with QE V6.4. It is affected only if 'code_num=2' (The codes will read vasprun.xml and CHGCAR from VASP calculations.) or 'lfock=.TRUE.' (calculation of the Fock exchange based on input Kohn-Sham orbitals.).
The total charge and spin-up spin-down charge difference are stored instead of spin-up and spin-down charge when code_num = 2.
fft_fact needs to be initialised before a one-step Fock exchange calculation using Kohn-Sham orbitals is done when lfock = .TRUE..
igcx = 31 for vdW-DF-cx0p was missing in gcx_spin_vec().