ground track table update can lead to KeyError in l3.build_dataset

[j-haacker]

Describe the bug
KeyError was thrown in l3.build_dataset when continuing gridding cached data chunks after the ground track table was updated.

To Reproduce
Steps to reproduce the behavior:

1. copy ground tracks to backup, read ground tracks, remove last one or more months of tracks, save the reduced data
2. delete the cache for some region (data/tmp/<o2-code>), run l3.build_dataset with some future end date
3. during gridding the cached data chunks exit the function
4. update the ground tracks or restore backup
5. continue processing (l3.build_dataset)

Expected behavior
Processing should continue with the old set of data. At least, it should continue in some way, e.g. including the recently added ground tracks, even if that leads to a slightly inconsistent result. At best, it would fill-up the missing time steps of the already-processed chunks and continue for the remaining chunks - resulting in an up-to-date, consistent dataset.

Traceback
not available :/

[j-haacker]

If you encounter this problem, revert the ground track data to the previous version and continue processing. You can easily do so with git checkout <commit hash> data/auxiliary/CryoSat-2_SARIn_ground_tracks.feather (assuming default setup).