Replace dune_add_library by target_sources

Simon Praetoriusrequested to merge
issue/replace-dune-add-library-object into master

Summary

The CMake function dune_add_library(<target> OBJECT ...) is deprecated and should be replaced by simpler functions like target_sources. See also dune-common!1037 for a detailed description.

Edited by Simon Praetorius

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