Merge GGK files, return the absolute path of the new database.

        Args:
            gswfk_file: Ground-state WFK filename
            dfpt_files: List of 1WFK files to merge.
            gkk_files: List of GKK files to merge.
            out_gkk: Name of the output GKK file
            binascii: Integer flat. 0 --> binary output, 1 --> ascii formatted output

def merge(self, workdir, gswfk_file, dfpt_files, gkk_files, out_gkk, binascii=0):
        """
        Merge GGK files, return the absolute path of the new database.

        Args:
            gswfk_file: Ground-state WFK filename
            dfpt_files: List of 1WFK files to merge.
            gkk_files: List of GKK files to merge.
            out_gkk: Name of the output GKK file
            binascii: Integer flat. 0 --> binary output, 1 --> ascii formatted output
        """
        raise NotImplementedError("This method should be tested")
        #out_gkk = out_gkk if cwd is None else os.path.join(os.path.abspath(cwd), out_gkk)

        # We work with absolute paths.
        gswfk_file = os.path.absath(gswfk_file)
        dfpt_files = [os.path.abspath(s) for s in list_strings(dfpt_files)]
        gkk_files = [os.path.abspath(s) for s in list_strings(gkk_files)]

        print("Will merge %d 1WF files, %d GKK file in output %s" %
              (len(dfpt_files), len(gkk_files), out_gkk))

        if self.verbose:
            for i, f in enumerate(dfpt_files): print(" [%d] 1WF %s" % (i, f))
            for i, f in enumerate(gkk_files): print(" [%d] GKK %s" % (i, f))

        self.stdin_fname, self.stdout_fname, self.stderr_fname = \
            map(os.path.join, 3 * [workdir], ["mrggkk.stdin", "mrggkk.stdout", "mrggkk.stderr"])

        inp = StringIO()
        inp.write(out_gkk + "\n")        # Name of the output file
        inp.write(str(binascii) + "\n")  # Integer flag: 0 --> binary output, 1 --> ascii formatted output
        inp.write(gswfk_file + "\n")     # Name of the groud state wavefunction file WF

        #dims = len(dfpt_files, gkk_files, ?)
        dims = " ".join([str(d) for d in dims])
        inp.write(dims + "\n")             # Number of 1WF, of GKK files, and number of 1WF files in all the GKK files

        # Names of the 1WF files...
        for fname in dfpt_files:
            inp.write(fname + "\n")

        # Names of the GKK files...
        for fname in gkk_files:
            inp.write(fname + "\n")

        self.stdin_data = [s for s in inp.getvalue()]

        with open(self.stdin_fname, "w") as fh:
            fh.writelines(self.stdin_data)
            # Force OS to write data to disk.
            fh.flush()
            os.fsync(fh.fileno())

        self.execute(workdir)

        return out_gkk