database.tables.display_entries: do not display microseconds

I updated the helper method ``display_entries`` from the database package which is used to display database entries in a nice ASCII table. While working on the 0.4 paper, I noticed that the microseconds cause a lot of clutter in the output, which *may* be more detailed than desired. This PR only display the year, month, day, hour, minute and second of each datetime column, not the microseconds. What do you think? Is it ok to leave out this information when displaying database entries or is it too important to be left out and "lies" because the output is not 100% correct anymore?