Added basic hypercube grid to bsl6d and added Kokkos into the build structure
I added a simple grid structure including IO to the bsl6d code including a mapping from a 6d to a 1d index.
Please review the code and overall grid structure and IO. (Usability and clean structure of code) This will not be the final grid structre. Refacturing topics are proposed in #5 (closed).
Tests have to be refactured. See #8 (closed).