MPL LpSolver/MipSolver backend

[Alpar Juttner]

The idea is to write an LP/MIP interface that is able to build up the problem, but when solving, it first create an MPL file, call the external command line solver, then parse back the output of the solver.

Of course it will be less efficient than using the C API of the same solver, but

• it can be useful to have the LP program built up by our code in MPL format, so that we can play with the solver separately (e.g. for experimenting with the tuning options).
• for some legal reason, it may simply be impossible to link the mip/lp solver to our code.