Draft: [feature][Grid] Add possibility to store user data pointers on the grid object.

@andreas.dedner: Surprisingly easy, yet effective. Also take a look at the corresponding branch in dune-fem.

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• 19f97cb1 - [feature][Grid] Add possibility to store user data pointers on the grid

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So, what is this?

This is a draft (as you can see) were I'm trying to explore the possibility to allow users to store data pointers on grid instances, which is some cases makes sense.