code: def ddk_filepaths(self):
          
          # This to support new version of optic that used DDK.nc
          paths = [ddk_task.outdir.has_abiext("DDK.nc") for ddk_task in self.ddk_nodes]
          if all(p for p in paths):
              return paths
  
          # This is deprecated and can be removed when new version of Abinit is released.
          return [ddk_task.outdir.has_abiext("1WF") for ddk_task in self.ddk_nodes]
docs: Returns (at runtime) the absolute path of the DDK files produced by the DDK runs.