Initial sensor calibration with Kalibr

Kalibr can be used to get ncamera calibration file necessary for maplab and ROVIOLI.

1. Create a new catkin workspace, clone and build Kalibr using the installation instructions from the Kalibr wiki.
2. Calibrate your sensor and obtain a Kalibr camchain yaml file. Check with the Kalibr wiki on how to do that. The relevant tutorials for a VI sensor: [1] camera calibration, [2] camera-IMU calibration
3. Convert the Kalibr output into the maplab format. Run the following command:

rosrun kalibr kalibr_maplab_config --to-ncamera \
    --label <your_label> \
    --cam <your_cam_chain_yaml>

where <your_label> is an arbitrary string to identify your ncamera setup and <your_cam_chain_yaml> refers to the file you receive from Kalibr in step 2.