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: )
Remove this check. `listAvailableDatasets` does not require the user to request any datasets.
It lists all that are available so the user can pick one to request later on when using `convertToText`