Add CMake guards for `dune-subgrid` dependency

Merged Alexander Jaust requested to merge fix-dune-subgrid-dependency into develop


Adds guards to CMake files to not build test cases and examples that depend on dune-subgrid if dune-subgrid was not found. This would be the case, e.g., if dune-subgrid is not installed.

Related to #28.


  • I made sure that the source files are formatted properly.
  • I added my changes to the changelog (

Merge request reports