close #235 - rename observer .hpp -> .param

Refactoring of radiation observer setup.

 - rename observer.hpp -> observer.param
 - add Doxygen documentation
 - add extensiv comments 

@ax3l: 
```bash
git mv ./src/picongpu/include/simulation_defines/param/observer.hpp ./src/picongpu/include/simulation_defines/param/observer.param
```
does not seem to result in a real renaming in git. (It's not file oriented.)