Create a Euro robocoin demo

- create a subdirectory in tutorials/ with a new notebook and a more detailed setup for an oracle
- fetch XTZ/EUR from a source
- create a few addreses and burrows
- provide helper functions in Python to help understanding the drift and index
- test Checker behaviour for a few attack scenarios

Start in the tutorials/ directory and plan the new notebook around the requested oracle setup. Define how to fetch XTZ/EUR, create addresses and burrows, expose Python helpers for drift and index, and cover Checker behavior in the listed attack scenarios. Done means the tutorial, setup details, helper functions, and scenario tests are all included.

Written by the indexing model from the issue text.