skip the generation of Benders binary cut if the bigM problem is unbounded

[yuxies]

Caused by unbounded \llv\ variables.

• If the bigM problem is determined unbounded or does not have an optimal solution, set the value of bigM to -inf.
• Add a boolean variable isBigMIncObjErr_ to track this case, and turn off Benders binary cut once the error is detected.

Will run more tests.

[yuxies]

Update:

• Changed the return type of findBigMBendersBinaryCut() to boolean, and the value of bigM is now passed using a parameter.
• If the bigM problem is determined unbounded or does not have an optimal solution, the function returns false.
• The Benders binary cut generator will be turned off if not findBigMBendersBinaryCut().
• Changed IncObj to BendBin in variable names.
• Keep using the variables bigMBendBin_ and isBigMBendBinSet_ to track whether the bigM problem has been solved (currently on $\mathcal(S)$) before and the bigM value has been recorded. The value will be stored in bigMBendBin_ and used for future generations of Benders binary cut.