Return the option string.

def _options_string(self):
        """
        Return the option string.
        """
        if len(self.options) > 0:
            s = ""
            for op in self.options:
                if self._sized_op:
                    s += "{:s}={:s} ".format(*map(str, op))
                else:
                    s += "{:s} ".format(str(op))
            return s.strip()
        else:
            return ""