Move reference data to the tutorials repository

[MakisH]

As described in the tutorials restructuring project:

In the future, we would also like to store reference results with each tutorial case, which could be used for the system tests. The vision is that in the future we only have tools in the systemtests repository, not reference results or case-specific patches.

[BenjaminRodenberg]

Do you already want to solve this within precice/tutorials#125 ? I like the idea, but would rather close precice/tutorials#125 soon than adding new features to the list.

[MakisH]

I would only do this afterwards, but it is related. Let's start closing boxes before opening more new ones.