Returns (at runtime) the absolute path of the DDK files produced by the DDK runs.

def ddk_filepaths(self):
        """Returns (at runtime) the absolute path of the DDK files produced by the DDK runs."""
        # 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]