Time limit for Gurobi LP Solver

[cxlvinchau]

This PR adds the possibility to specify a time limit when using the Gurobi LP solver. This allows for stopping the solver before an optimal solution is found and retrieving sub-optimal solutions.

The following changes have been made:

• GurobiEnvironment now has a setTimeLimit(uint64_t seconds) method that sets the time limit parameter of the underlying Gurobi environment.
• Accordingly, GurobiLpSolver has a setTimeLimit(uint64_t seconds) method that sets the parameter in the GurobiEnvironment.
• The methods for retrieving sub-optimal solutions have been adapted, i.e. getContinuousValue(...), getIntegerValue(...), getBinaryValue(...) and getObjectiveValue(...). More precisely, the checks for infeasiblity and unboundedness have been removed. If the problem is infeasible, then the check of the solution count will be zero. If the problem is unbouded, Gurobi might still find a solution before it determines that the problem is indeed unbounded. In that case a user might still be interested in a solution.