Install fix

Closed Edscott Wilson Garcia requested to merge edscott.wilson/dumux:install-fix into releases/3.1

**This MR fixes the "make install" instruction / DuMux needs it so user can do a standard make install without having to hack source **:

**Which issue this MR fixes: Fixes install references to non existing files.

Special notes for your reviewer:

I think I have the merge request done correctly by updating my local fork to the current releases/3.1 branch.

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