Use Nexus files for dream workflow instead of geant4

Would be nice to use a Nexus file (like the ones @celinedurniak made for a stap demo a while ago) which is basically simulation data inserted in a Nexus file.
So that we can use the GenericNexus workflow instead of the geant4 workflow.

Note that we are using some nexus files in the tests, but it would be nice to do the same in the docs.