Add scan-timestamp for dataloaders we know already this value (TUM poses format)

[nachovizzo]

We added the TUM pose format to the results of the python pipeline (#45) But we haven't extended this to all other existing datasets besides tum.

The idea is rather simple, take as an example the rosbag, rosbag2, mcap dataloaders. For each scan, we already know the timestamp of that particular sensor frame (not to be confused with point-timestamps). This is the time we recorded the scan.

We should store that value for each scan, and take advantage of the tum poses format to produce a result which is more meaningful for some applications.