[Cmake] prevent headercheck from polluting the cache

Cmake by default added a dependency variable for each headercheck
target to the cache. As these dependency should not be subject to
change by the user this is unwanted behaviour. Additionally, the
amount of headercheck targets pollutes the cache. The patch fixes
this by removing the cache variable.
[Test] add a small test that uses the Dune::InitAllocator
Update the allocator and its doxgen docu