Addition of external ionic force field to pw.x and cp.x

The modifications allow for applying external ionic force fields at each iteration of a MD run in pw.x and cp.x (or also during a force relaxation). A new system variable is defined (nextffield), which sets the number of activated forcefields. The definition of each force field is contained in an external file extffield.dat. See Modules/extffield.f90 for more information.