Output Ouster timestamps on python pipeline.

[pierrebenet]

Hello,

I needed to output timestamp of the trajectory of ouster SLAM on tum_poses.txt.
I had to do a few modification to do so. I think this would be useful for others.
This can be easily done by these modifications on python/kiss_icp/datasets/ouster.py :

[nachovizzo]

Then open a PR! That would be of great benefit to the community, and your first open-source contribution ;)