[BUG] PowerMethod is not implemented right.

[paquiteau]

Describe the bug
The PowerMethod algorithm implemented in math/matrix.py is not right, we are using the old norm to divide, and not the norm of the new estimate of the eigen vector.

see here

 x_old = self._set_initial_x()

        # Iterate until the L2 norm of x converges.
        for i_elem in range(max_iter):

            xp = get_array_module(x_old)

            x_old_norm = xp.linalg.norm(x_old)

            x_new = self._operator(x_old) / x_old_norm // <- should be x_new_norm

            x_new_norm = xp.linalg.norm(x_new)

The Pull Request is on the way.