Feature: Convert Kraus operators to the natural representation

[vprusso]

Feature: Given a list of Kraus operators, calculate the natural representation of the corresponding quantum channel in the standard basis.

For the definition of the natural channel representation, consult Section 2.2.2 in TQI.

Something akin to the following would likely

import numpy as np
from toqito.matrix_ops import tensor

def natural_representation(kraus_ops):
    return np.sum([tensor(k, np.conjugate(k)) for k in kraus_ops], 0)