add an option to write optimal value and solutions to a file

[yuxies]

• If MibS_writeInstanceName followed by a string name is detected, the optimal value and solutions will be written to the given file name.
• If there is no input following the parameter or the parameter is not used, results will be displayed through std::out.

The output will look like this:

============================================ Optimal solution: Cost = -2 First stage (upper level) variable values: C0000001 = 1 C0000002 = 1 C0000003 = 1 Second stage (lower level) variable values: C0000004 = 1 C0000005 = 1

[tkralphs]

OK, I made it so the solution always prints to the screen, which is I guess what we want. Looks good!