Cleanup the cmake structure

Simon Praetoriusrequested to merge
feature/cleanup_cmake into master

Summary

Replace individual object libraries by common dimension-dependent library duneuggrid_xd and one exported target duneuggrid

Details

all source files are either added to duneuggrid_xd or to duneuggrid using target_sources command from cmake. This allows to just define one exported target that can be imported by other dune modules.

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