Function verify_subadditivity 

pub fn verify_subadditivity<FJoint, FMarg>(
    joint_entropy: FJoint,
    marginal_entropy: FMarg,
    x: &[u8],
    y: &[u8],
    tolerance: f64,
) -> bool
where
    FJoint: Fn(&[u8], &[u8]) -> f64,
    FMarg: Fn(&[u8]) -> f64,

Verify subadditivity: H(X,Y) ≤ H(X) + H(Y).

This is equivalent to I(X;Y) ≥ 0.