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: )
Why do you print values of scalars in the `listAvailableDatasets` method? As the name suggests, it should only *list* datasets. All values should be printed in `convertToText`.