First step to XC library (LDA for now)
The xc drivers for LDA and LSDA have been put into xc_lda_lsda_drivers.f90. The exchange and correlation functionals are in separated files too. Also the derivative of xc functionals drivers (dmxc- routines) have been put in a different file, dmxc_drivers. Now the routines (both drivers and functionals) take, as input, arrays of chosen length. All has been tested, but more checks are welcomed. A simple benchmark test (benchmark_libxc.f90) with libXC library is available in PP/src folder.