This MR allows for deeper hierarchies in CMake. Currently,
CMakeLists.txt files are only supported on the first level of the base directory, e.g.
/src/module/CMakeLists.txt. With this MR, we also allow deeper hierarchies with submodules, e.g.
In the current waLBerla implementation, this is not directly needed, but in some applications, this feature might come in handy.