There are too many interface functions computing exchange and correlation.
Functions exc_t, vxc_t moved out of module funct to the only package where 
they are used (atomic). They should be replaced by xc, xc_spin sooner or
later. 


git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11084 c92efa57-630b-4861-b058-cf58834340f0