Allow State.degenerate_decouple

[zerothi]

Describe the feature

Up till now the decoupling algorithm was only present in the derivative method.

But, it does not belong there.

Instead one should dis-entangle the states through some kind of inner-product matrix with an operator.

The procedure could be something like this:

es = H.eigenstate(...)
# closewithin 1e-5 eV
degenerate = es.degenerate(1e-5)

for deg in degenerate:
    der_xyz = es.sub(deg).derivative(matrix=True)
    der = der_xyz[0] * 0.5 + der_xyz[1] * 0.5
    es.state[deg] = eigh(der)[1].T @ es.state[deg]