adding a method to get the coefficient of the variable in the objective function.



See merge request !2

merging with lbjava's ilp-solver interface

Changes: 
- merges the two `ILPSolver` interfaces (in this package and the one in LBJava)
- merging the two gurobi solvers:  for most of the syntax I have used that of LBJava's. 

If this get's merged, we can remove `IlpSolver.java`, `GurobiHook.java` and `oJAlgoHook.java` from lbjava and add this `illinios-inference` as LBJava's dependency.  

FYI @mssammon 

See merge request !1

  + merging the two gurobi solvers

Updated dependencies
Removed CuttingPlaneILPHook

moving to v.0.4.2

Made Gurobi an optional dependency.