Implemented vdW-DF-cx from K. Berland and P. Hyldgaard in Modules/funct.f90 and flib/functionals.f90 - see PRB 89, 035412 (2014)

git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11102 c92efa57-630b-4861-b058-cf58834340f0