Disable translation estimation when calibration

[ClementLeBihan]

Is your feature request related to a problem? Please describe.
The final camera_lidar translation is wrong, but the projection is pretty correct. As I already know (+- 5cm) the position of the camera, I would love to do a first step of optimization computing only rotations. And when we reach the best estimation, start from this to refine the translations+rotations. I hope it will decrease the translation error.

Describe the solution you'd like
I would love to know how to force the optimizer to don't optimize translations. Only rotations.

[koide3]

I should be possible by adding a factor that constraints the translation components around the initial position at the following optimization part.

direct_visual_lidar_calibration/src/vlcal/calib/visual_camera_calibration.cpp

Line 216 in 8a03c73

ceres::GradientProblem problem(cost, new Sophus::Manifold<Sophus::SE3>());