Status of the solution. Before solving a problem the status will be NOT_SOLVED;
afterwards it will take any of the other values depending if it successfully found a solution and
if the solution is optimal.
Status when an optimal solution has been found.
Status when a feasible (not necessarily optimal) solution has been found.
Status when the current model is unfeasible (has no solution).
Status when the current model is unbound.
Status when it failed to find a solution for unexpected reasons.