[ug] Don't use dune-uggrid's symlinks

Christoph Grüningerrequested to merge
feaure/dont-use-ug-symlinks into master

Use proper include paths instead

This breaks compatibility with UG 3.13, not dune-uggrid.

Requires staging/dune-uggrid!143 (merged)

Edited by Christoph Grüninger

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