Public Headers: openPMD/Serial.hpp openPMD/Parallel.hpp

Should we replace our public header `openPMD/openPMD.hpp` with an explicit serial or parallel version? `openPMD/Serial.hpp` and `openPMD/Parallel.hpp`?

And should it be possible to include both in the same translation unit and how do we avoid ODR issues in multiple translation units?

This would avoid having users to set macros and allows more seamless usage of our parallel features with serial usage.