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: )
what do you use `inputSize` for? why do take the size of a `std::string` and use it in the `for` loop?