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.

まず、BQM から posiform、fixed variables、implication graph、max flow へと続く roof duality のフローを追跡します。float-to-integer および integer-to-float 変換を行わずに、元の posiform がどのように lower bound を生成できるかを、roof duality が固定しない変数も含めて特定します。数値的により正確な lower bound が得られれば完了です。

索引モデルが issue の本文から書いたものです。