GGA vectorized and libxc functionals included

GGA part of XC vectorized and put into xc_gga_drivers.f90 Libxc library incorporated for GGA. q-e now recognizes libxc functionals by their name an automatically use them (if libxc is active). Derivatives of xc functional are also available in libxc. The derivatives for the gga part have been vectorized, but still need some work in order to use libxc. A complete benchmark testing code between qe and libxc is available in PP/src/benchmark_libxc.f90, including xc potential derivatives. However deeper tests are required in order to clarify numerical discrepancies between libxc and qe in some cases.