This function computes the largest integer value not greater than val.

Examples:

 IloFloor(IloInfinity) is IloInfinity.
 IloFloor(-IloInfinity) is -IloInfinity.
 IloFloor(0) is 0.
 IloFloor(0.4) is 0.
 IloFloor(-0.4) is -1.
 IloFloor(0.5) is 0.
 IloFloor(-0.5) is -1.
 IloFloor(0.6) is 0.
 IloFloor(-0.6) is -1.