Reorder initializer list

This should fix the following compiler warning:

| .../quadraticipopt.hh:707:41: warning: ‘QuadraticIPOptSolver<...>::constraintMatrix_’ will be initialized after [-Wreorder]
| .../quadraticipopt.hh:703:89: warning:   ‘... QuadraticIPOptSolver<...>::constraintObstacles_’ [-Wreorder]
| .../quadraticipopt.hh:652:5: warning:   when initialized here [-Wreorder]