Calculate more numerically accurate lower bound.

In roof duality, we convert a bqm to a posiform , convert its coefficients to integer coefficients, then convert it to an implication graph and find the max flow. We use the max flow to compute the lower bound of the bqm, but we may be able to take the original posiform use the fixed variables and find the lower bound. This will bypass the float-to-integer and integer-to-float conversions and provide a more accurate lower bound. We need to find out a way how to use the original posiform since roof duality will not fix all the variables in the posiform.