calibration: make max_intensity and min_intensity optional (#84)
calibration: make max_intensity and min_intensity optional (#84)

This fixes a regression in the 32e and VLP-16 calibrations which do not contain
intensity values. There is still a problem with the 64e_s2.1 calibration.

lemiant/velodyne,lemiant/velodyne,wenjiebit/velodyne,vrabaud/velodyne,wenjiebit/velodyne,lemiant/velodyne,vrabaud/velodyne,vrabaud/velodyne,wenjiebit/velodyne