Decide wether the handler should overwrite existing output

[JanHammelmann]

We should decide on how we run the software and overwrite already existing results. E.g. in smash one can run the code with the option -f to force overwriting. In vhlle or in the sampler I don't know how this is done. In the current cmake structure if the output already exists, one cannot re-run code to the same output directory. A solution could be to add an option to the HybridHandler that allows for overwriting but for now I suggest to exit the handler if the output already exists.

[JanHammelmann]

As we discussed in the sprint, we will wait for the outcome of smash-transport/new-hybrid-era#86 before continuing with the implementation of this option.