Deeper hierarchies in CMake.

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. /src/module/submodule/CMakeLists.txt.

In the current waLBerla implementation, this is not directly needed, but in some applications, this feature might come in handy.