fix exportPoses() in camera-imu calibration

[Youras]

fix related to issue #577
Setup: ROS noetic - Ubuntu 20.04.

[ethzasl-jenkins]

Can one of the admins verify this patch?

[goldbattle]

I have added the ability to export poses to the kalibr_calibrate_cameras so that both match.
Additionally, have added the ability to see the trajectory in the pdf / plots at the end of calibration for both.
Should help people debug a bit, I hope.

[goldbattle]

Thanks again for the PR.