Return the transmon wave function in number basis. The specific index of the
wave function to be returned is which.
Parameters:
esys (Tuple[ndarray, ndarray]) – if None, the eigensystem is calculated on the fly; otherwise, the provided
eigenvalue, eigenvector arrays as obtained from .eigensystem(),
are used (default value = None)
which (int) – eigenfunction index (default value = 0)