Import built grid tutorial slides

Dr. Jorrit Fahlkerequested to merge
grid-tutorial into master

Built from https://gitlab.dune-project.org/pdelab/dune-pdelab-tutorials master@e0a8467a53993f32c623c544a5aedaffa19992a7

Closes: #22 (closed)

Edited by Dr. Jorrit Fahlke

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