Export solver primal/dual objective value

[mtanneau]

This PR

I had to add a safeguard for dealing with NaN / Inf values in primal/dual objectives. A solver may returned those when an instance is primal or dual infeasible, or when an error is encountered during solving. In such case, the primal/dual objective value in the result dictionary may be NaN, Inf or -Inf

However, JSON does not support those values. As a consequence, when reading the JSON file from disk back into memory, all such values will read as nothing. Notes:

• At present, this only seems to be an issue for ACOPF because the dual objective value is not defined (the problem is non-convex)
• We might have issues when using Gurobi to solve quadratic relaxations (because it doesn't always support querying dual information), and possibly when a convex instance ends up being infeasible or unbounded (depending on what the solver would report as primal/dual objective value).
• In any of the above case, the user would have access to termination status and primal/dual statuses without loss of information.

[ivanightingale]

It seems that you still have res["objective_lb"] = -Inf in all the formulations instead of setting it to the dual objective value. Is this intended?

[mtanneau]

It is not intended. Thanks for catching that.