MetaGGA routines in array form and libxc included

In line with the work done for LDA and GGA, the metaGGA routines have been put into 'array form' and libxc routines have been incorporated. Now metaGGA functionals from libxc can be selected from input. The file PP/src/benchmark_libxc is now updated. Some discrepancies are present between q-e and libxc in the correlation part of the potential for m06l functional. The initialization routine for xc functionals (init_xc) has been removed.