Topic adiostotxt

According to Issue [#299](https://github.com/ComputationalRadiationPhysics/picongpu/issues/299) I added the possibility to use splash2txt to print out the content of your adios files to txt files. (thanks to f-schmitt for your help :smile: )
It seems to me there is a lot of code duplication here by writing two `switch` statements.
You should create a seperate function instead (e.g. `printValue`) which takes the ADIOS variable type and the pointer to cast.