CERN 2020 version

What's Changed

• adding a snippets folder by @pfs in #1
• factorizing classification/training notebook by @pfs in #2
• adding some more tasks to the notebook by @pfs in #3
• minor edits + ABCD method notebook by @pfs in #4
• adding full shape analysis example by @pfs in #5

Full Changelog: https://github.com/CMSDAS/pps-long-exercise/commits/CERN2020