The problem is, if we include ```*.tpp``` inside ```*.hpp``` than we can not use e.g. the definition of the class inside ```Particles.hpp``` before all dependencies inside ```Particles.tpp``` are fully solved. 
This is a big problem which can not solved with ```*.def``` constructs or similar.
If we have a clean ```*.hpp``` file we can use the class Partciles inside PIConGPU, can call method before the method by it self is implemented and all dependencies are solved.