Processing Poolsolutions (nearoptimal)

[leroum]

What

In the context of processing the results of #1138 it would be nice to add that you can also process the results you get from gurobi_generate_solutions.

Why

With this feature you get not only the one optimal solution, but also near-optimal solutions, which is very useful especially for design optimisation and multi-period optimisation.

How

As far as I could grab it, the reason why the results processing does not work now with the solution list from gurobi_generate_solutions is because the results processing now works only with the model (but here only the one optimal solution is stored) and the return from grobi_generate_solution is a solution type.

Maybe this could be a nice project for the next dev days if enough people are interested. (If you need more information about near-optimal solutions to decide if you need this feature, please let me know)