Traits for splash types

Add some type traits to convert our units to libSplash [CollectionType](http://computationalradiationphysics.github.io/libSplash/class_d_collector_1_1_collection_type.html)s and back.

I did not add the trait to `HDF5Writer.hpp` to avoid merge conflicts with René's up-comming `flex-particles`.
Furthermore, we should honour the stored type and try a host-side TypeCast during `restarts`.

I will use the proposed traits first with a rebase in my [topic/phase-space](https://github.com/ax3l/picongpu/tree/topic/phase-space) branch.