Create a .tar archive before copying a checkpoint folder to Google Drive

[woctezuma]

#34

Caveat: I have not tested the changes.

[woctezuma]

I have tested it myself, and fixed the tree structure which was not preserved at first.

I chose not to delete the .tar archive file from the Colab machine:

• it would be deleted automatically anyway once the Colab session times out,
• if a user calls the function several times, it would be overwritten, so the disk space is identical.