Add helper function to construct a bell pair with fidelity F

[hanakl]

The following helper function is currently used in several examples throughout the codebase, and it would be nice to have it defined.

function noisy_pair_func(perfect_pair, F)
    p = (4*F - 1) / 3
    perfect_pair_dm = SProjector(perfect_pair)
    mixed_dm = MixedState(perfect_pair_dm)
    return p * perfect_pair_dm + (1 - p) * mixed_dm
end

[Krastanov]

I think it would make most sense to have this as a symbolic object that relies on QuantumSymbolics.jl to be expressible in arbitrary formats. We should probably have it in the StateZoo and call it something like DepolarizedBellPair or WernerBellPair with optional constructors that use either the keyword fidelity or the keyword depolarization_probability, with a nice docstring explaining the difference.