Snapshot testing util does not work with datatime index

[AKuederle]

When trying to save a pandas df with a datetime index with the snapshot test, the roundtrip does not work, as pandas seems to round the timestamps before saving.

A workaround is to round the timestamps to ms before applying the snapshot, but we should just directly integrate this workaround in the testing tools

[AKuederle]

Had to reopen the issue, as I am still having trouble with that for some ns-timestamps. Will investigate further.
For now old workaround is still valid

[a-mosquito]

Maybe this is related to this issue?

[AKuederle]

Actually fixed by #112