Add CMake guards for `dune-subgrid` dependency
Description
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.
Checklist
-
I made sure that the source files are formatted properly. -
I added my changes to the changelog ( CHANGELOG.md
)