[mm] fix link to grids in source directory

Merged Simon Emmert requested to merge fix/mm-link-to-grids into master

We somehow removed the link to the grids for some exercises. Since the links are provided in the CMakeLists for the tests, the tests still pass, but students cannot run the problems without manually copying or adding the link to the grids.

Hence I included the link to the grids in the CMakeLists

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