Consider adding a method that measures the "frustration" of a BQM

[arcondello]

"Frustration" is in some sense a measure of problem hardness. That is, what is the energy contribution of the linear and quadratic biases that are violated by a given solution.

Something like

import dimod

def frustration(bqm, sample):
    frustration = 0

    for v, bias in bqm.linear.items():
        frustration += max(0, sample[v] * bias)

    for (u, v), bias in bqm.quadratic.items():
        frustration += max(0, sample[u] * sample[v] * bias)
    
    return frustration

if __name__ == "__main__":
    bqm = dimod.generators.gnp_random_bqm(10, .5, "SPIN")
    sampleset = dimod.ExactSolver().sample(bqm)
    sample = sampleset.first.sample
    print("energy:", sampleset.first.energy)
    print("frustration:", frustration(bqm, sample))