Feature: PBR (Pusey-Barret-Rudolph) states

[vprusso]

The PBR basis, defined in arXiv:1111.3328, is an interesting and often useful basis of states.

These states are defined in Equation-A6 in the Appendix. One approach to define

def pusey_barret_rudolph(n: int, theta: float) -> list[np.ndarray]:
    """Equation (A6) from arXiv:1111.3328
    
    Basis of 2^n states parameterized by angle ``theta``. 
    """
    dims = n * [2]

    psi_0 = np.cos(theta/2) * e_0 + np.sin(theta/2) * e_1
    psi_1 = np.cos(theta/2) * e_0 - np.sin(theta/2) * e_1
    psi = [psi_0, psi_1]

    binary_strings = list(itertools.product([0, 1], repeat=n))

    states = []
    for b_str in binary_strings:
        state = []
        for b in b_str:
            state.append(psi[b])
        states.append(tensor(state))
    return states

Should add these to states/pbr.py or states/pusey_barret_rudolph.py.