Method observations

examples/bal_problem.h:76–76

  const int* point_index()     const { return point_index_;              }
  const int* camera_index()    const { return camera_index_;             }
  const double* observations() const { return observations_;             }
  const double* parameters()   const { return parameters_;               }
  const double* cameras()      const { return parameters_;               }
  double* mutable_cameras()          { return parameters_;               }