Make the directory exist, then touch the file

        If the filename is None, then use self.name as filename

def make_file_exist(self, filename=None):
        """Make the directory exist, then touch the file

        If the filename is None, then use self.name as filename
        """
        if filename is None:
            path_to_file = FilePath(self)
            path_to_file.make_file_exist()
            return path_to_file
        else:
            self.make_directory_exist()
            path_to_file = self.touch_file(filename)
            return FilePath(path_to_file)