Convert gate tensor to the special unitary group.

def su(self) -> 'Gate':
        """Convert gate tensor to the special unitary group."""
        rank = 2**self.qubit_nb
        U = asarray(self.asoperator())
        U /= np.linalg.det(U) ** (1/rank)
        return Gate(tensor=U, qubits=self.qubits)