Deprecate support for SuperLU 4.x

Removal of older SuperLU versions to reduce the number of combinations to check. SuperLU 5.0 was released in July 2015. If we deprecate SuperLU 4.x in Dune 2.7, it will be removed in 2020 with Dune 2.8.

added 13 commits

• 430a2585...85fce383 - 12 commits from branch master
• 873e7699 - Deprecate support for SuperLU 4.x

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enabled an automatic merge when the pipeline for 873e7699 succeeds

merged

mentioned in commit 7ea53326