zhixy

I notice that the number of threads is set according to cpu core [here](https://github.com/ethz-asl/kalibr/blob/14e47ea0edb7a42efa03533855e533f9e6ad7bb8/aslam_offline_calibration/kalibr/python/kalibr_imu_camera_calibration/IccCalibrator.py#L158), however when I really calibrate camera-imu, only one thread is been used as I make sure...